Comment 2 for bug 300870

I had this problem with Ubuntu 8.04 (nvidia-glx-new) and currently with Ubuntu 8.10 (nvidia-glx-177).
I'm really more concerned with the problem illustrated by gksu. It's not just that particular dialogue though. It also occurs with the shutdown dialogue and various other things like Select Session in GDM. Also, the default GDM login themes are centered like that as well, however I managed to fix that by setting the position to 25% in the theme xml file as a opposed to 50%