Thanks a lot Lethe.
Sadly in my case disabling RenderAccel is not an option as I use most of the times my linux box with Gnome (I gave up using it with KDE4 together with NVidia cards long time ago...).
I remember that once I had this freezing on GDM screen, so I think that (at least this time
) compiz is not guilty.
Thanks a lot for your help,