and I should add ‘only’ when gdm uses gnome-shell login screen. This problem (probably) appeared with Gnome 3.4, and I first noticed on ArchLinux several months ago. Now it affects openSUSE 12.2 as well … and googling for new answers, all I found was my old question. 
$ aterm
X Error of failed request: BadWindow (invalid Window parameter)
Major opcode of failed request: 20 (X_GetProperty)
Resource id in failed request: 0x100000b
Serial number of failed request: 26
Current serial number in output stream: 26
More info here: aterm doesn’t start under twm/ctwm/mwm (only)
I would appreciate not to receive the same kind of answer I got on the Arch forum - which is otherwise very helpful. 
I hope it could be fixed with some X server option … will try a couple ones (provided gdm doesn’t ignore them).