It would be *totally* cool to have theme support for this, so you can
do something like the window at the bottom of Apple's iSync page:

  http://www.apple.com/isync/

:-)

(disregard that comment, it's more appropriate for a different bug)

Gaim's notify plugin will set the urgent hint when a new message is
received and not yet replied to (or until the window gets focus, or
you start typing... it's got a few options).  Anyway, it'd be cool if
metacity supported it somehow.

This seems to me like it would be best handled in the panel.  Windows
has something like this by flashing the box on the taskbar when a
window wants attention but you're not giving it to it.  This seems
like a useful UI feature.  I'm not sure though if this would require a
EWMH augmentation or it the panel could see the Urgency hint on its own.

Agreed, I'm not sure what metacity should do with this hint. 
There may be a libwnck bug already for tasklist to do 
something with it.

I think the windows behavior is pretty good here, but we need to make
sure this is done conservatively in applications. Lots of flashing on
the window list would suck.

See libwnck Bug 99679

*** This bug has been marked as a duplicate of 99679 ***