There is also a Realtime (RT) kernel available in Ubuntu Lucid, but it is currently based on the older Linux 2.6.31 kernel. We were unable to test out this real-time kernel due to DRM problems with the Linux 2.6.31 kernel and our newer ATI Radeon graphics card.
Michael, can you give a little more information this issue? Was it an Ubuntu-specific problem or is there a more general conflict?

I'm currently running on with an HD4670 and I've been thinking of upgrading at some point this year to a newer kernel (.31 or .33), so I'm curious if there are rough waters ahead for me.