The action buttons (Log In, etc.) on the gdm dialog don't work when the dialog is first displayed. I think this happens when my mouse pointer starts out in the same position as one of the buttons will be drawn. In this case, we get half a hover effect (it kind of turns 3D), but it doesn't light up with a blue background. Clicking does nothing. I have to move the pointer out of the button and then back over it before it will work. gdm-2.22.0-10.fc9.i386
The particular case was actually the Shut Down button, not the Log In button, and the effect is a blue highlight but no border (so I think it's indistinguishable from the correct hover state visually).
An anonymous coward claimed that this bug was fixed in today's release of Ubuntu (http://news.slashdot.org/comments.pl?sid=1012917&cid=25571255), so it's probably fixed in the Rawhide version of GTK2, just a question of finding the upstream bug and proving that it's fixed.
It's http://bugzilla.gnome.org/show_bug.cgi?id=56070
Yup, should be good to go for F10.