The unit is the image's unit. The global setting from
the preferences dialog is the unit used for newly
created images only.

Changing the image's units (using image->scale) is
effective immediately in the info window.

That works, but is hardly intuitive, when you want to change
temporarily from one unit to another.
I'm learning that Gimp places some of its options in the most peculiar
places. Setting and changing image units under the Scale menu is
hardly logical. I suppose it's too much to ask to have the Ruler units
set in Preferences (quite apart from New Image units), and to have a
simple way to change the current units, say by right-clicking on the
ruler to get a small contextual menu offering units, maybe the same in
the Info palette too. 
Is it worth another bug report, now you've closed this one?

Right, changing the image's unit using "Image -> Scale" is
not what I would call intuitive :)

Reopening this bug as enhancment so we don't forget
about the issue.

Note it's not as trivial as just changing "the ruler's" units
because units get used in lots of places when dot-for-dot
mode is disabled: in the coordinates display, in the tool
options and in many dialogs. All dialogs have unit menus
but they are all initialized from the image's unit at the
time the dialog was opened.

So if we go for separating the image's unit from the unit
this single display is currently using, we also need to
decide which of the places which initialize themselves
from the image's unit will need to use the display's unit.

IMO the unit should be bound to the image. But it surely makes sense
to add ways to change the image unit in a more obvious place.

IMO the place for stuff like Ruler units is Preferences, and any
change to Prefs take effect immediately, universally. At the moment, I
set a Grid Offset, it won't take effect until restart. 
You've got preferences for Default Grid, is that not the same as the
ruler units? I haven't looked too closely into this. 
As for Info display, surely it's just a question of applying a
conversion formula to whatever is the image's units? It is merely a
guide to the user. Similarly if you had a contextual manu on the
Ruler, the image would still be based on said unit, and new images
defaulting to this as set in Preferences, but for display purposes
only the units were changed on the Ruler, and elsewhere it would still
use the document's real units. Maybe one option on this hypethetical
menu is Default, which switch back to the image's actual units. The
ruler too is only a guide anyway. If the default is Centimeters, and I
want Millimeters for something closeup, then the conversion is
straightforward. Pixels is the only Real unit of a digital image
anyway, esp on the screen. Inches, millimeters etc is subject to
Resolution, the density of pixels. Change the resolution, and the
Image dimentions change too, in reverse proportion, but the actual
image size (in pixels) is the same. No actual image data has changed.
I don't see a need for a distinction between each individual Image's
Unit and universal units, as long as the Universal units can be
changed with immediate effect, otherwise some sort of Image properties
submenu would be needed. As I said, the Ruler (and Info) units would
only be a guide, and the only place where the selected units are in
effect. To change all the dialog's units, go to Prefs. No-one's going
to check if their color printer is accurate within half a millimeter
of the stated Ruler/Grid unit. For anything more accurate, use a
layout program to position things. 

Incidentally, the ruler doesn't indicate which unit is in effect.
Maybe having "mm", "px", "in", etc near the Origin next to the 0 would
help? Small thing, but may avoid confusion for someone. It's not like
the Real World where size is fixed. I resize the image, and the scale
of the units changes too. I have heard of people who put rulers to
their screens to check the size of an image =]

PS: You can tell I'm from Europe (not England), I think in Metrics :)

This really is an enhancement request since there is nothing wrong
with the current behaviour. Changing back.

Changing the unit in the preferences is not possible. The unit is
bound to an image. It is stored with the image and it is part of it.
Settings in the preferences dialog can only be applied globally but
there's nothing global about the image unit. You want to be able to
work on images with different units at the same time.

Also there is an indicator for the image unit. The unit is shown in
the lower left corner where the mouse coordinates are being displayed.
Perhaps we should make it show "px" in Dot-for-Dot mode. It would also
be a good place for a context menu to change the image's unit or
perhaps even the display unit if we want to introduce such a concept.

And of course, an image property dialog is absolutely needed and is
planned already.

Just so that everyone is clear - it is possible to set the default
unit for images in the preferences in the same way as you can for the
grid. 

Both the length unit and the grid are per-image things. Both are
changeable dynamically, but the image length unit is harder to find. 

Dave.

I wan't sure if an Image Dialog was necessary, as the Info window
already shows details for it, except of course it doesn't allow to
change anything. The only thing I could think of for the Image Dialog
that wasn't encroaching on another dialog, like the Scale dialog, was
Units, and perhaps Resolution, which can be done from Scale as well.
Perhaps this Image Dialog will be a better home for the Print Size &
Display Unit section currently in Scale. Seems Document-specific. On a
Mac you can add comments to files via Metadata (perhaps an enhancement
for MacGimp), such as where a photo was shot etc, don't know about
Unix, certainly Windows is a stranger to Metadata, at least until
Longhorn. I suppose any of the metadata in Digital Camera images
(EXIF) will go here too (don't have one, but I gather there's details
of the camera settings, time taken, etc, all valuable to photographers).

This is a separate thing, but I notice Scale and Canvas shows Ratio.
Ought this not be Percentage? Would that not make more sense to users?
Most graphics packages, most notably Photoshop, use Percentage. Should
I make a separate Enhancement Request for that?

I don't think percentage is any better than ratio, so I don't see a
compelling argument to change a long-standing and well-established
GIMP user interface element.

Of course a proper image property dialog could replace the info
window. Or the functionality could be added to the info window... But
since we want property (or attribute) dialogs for layers, channels,
vectors and more, I'd say we get rid of the info window or make the
info window a generic container for property dialogs. 

The General tab of the Info window can happily be moved to the Image
Property dialog, but please don't even consider removing the Extended
info tab, which shows interactive info on color etc. For image
editing, such as color correction using the Levels/Curves, it is
essential to have Info open to observe the highlight/shadow/neutral
values. A monitor cannot be trusted. Whether it is tabbed with Image
Properties depends on how big this property dialog is. Info needs to
be discreet enough not to get in the way. In that sense the current
one could be more space efficient for the data it shows.

Discussion of an Image Property Inspector doesn't really belong here,
so since I didn't find an existing report on it, I created a new
enhancement request where suggestions can be made:
Bug #137779

With current CVS the unit can be changed in the image display. Should this
report be closed or do we keep it open until the image properties dialog is done?

IMO the main point of this report has been addressed with the prominent unit
menu at the bottom of the image display. Closing as FIXED.