So the two things that use the Notification Applet in the 2.1.x series
don't actually use it for the Forces of Good. We have:

  - acme: Displays an icon to show that the acme daemon is running,
includes menu with about/preferences/etc. Doesn't do any notification. None
of our other user daemons have notification icons! :-)

  - cd player: Displays an icon to show that the cd player is running (in
fact, the cd player window doesn't pop up when you run it, you just get the
icon, which is a bit confusing), includes menu with
play/next/prev/prefs/about/etc.

  - There are a number of other things that use it outside the desktop
release, such as GnomeMeeting, Rhythmbox, etc. Not many use it correctly.

When Havoc first proposed the new applet, I was of the understanding that
it would only be used for notification icons. When something happens, an
icon pops up for the user to see... and instant message comes in, a new OS
update is available [1], etc. Currently it's being used the same way as the
Windows system tray (completely randomly, mostly for the purposes of
software pimping), and at some stage we'd have to implement icon hiding for
the stuff that doesn't do any notification.

So, it would be good to get some guidelines in the HIG... Happy to thrash
out some stuff in this vein.

[1] Even RHN keeps an "OK" icon there all the time. It should really just
pop one up when new stuff is available, or an update is taking place. I
guess that's a marketing decision though. :-)