I returned to Kubuntu the other day and all has been fine accept for KWin's OpenGL mode. When using it, if any other software running uses OpenGL, it gives a high chance of causing the driver to hard lock (everything graphical freezes). This is usually the case when I have Second Life running and I resize or maximise a window or change window decorations. These usually have to be done quite a few times before it happens but the odd resize or maximise will lock the graphics up. When I don't have Second Life running, there's no problem.

I'm currently testing if XRender causes the same problem, but so far it's been alright.

I have a 9800GT 1GB graphics card, Kubuntu 11.04 up to date, NVIDIA 270 driver (from additional drivers) and using TwinView to run two monitors.

Would this be more of a bug in the NVIDIA driver or KWin doing something very bad?

EDIT: Graphics hard locked on XRender after switching window decorations. Now I'm going to have to just leave compositing turned off. :/

EDIT 2: Well, NVIDIA 275 has a fix for the problem. Currently testing.

Yes, this is a bug in the graphics drivers (which itself is actually a bug in DRI, the 3D acceleration system being designed for only one client. DRI2 can handle more than one)