Well, I managed to upgrade to Ubuntu Edgy 6.10, though the path to this upgrade was a bit rocky. I've tried to get the ATI proprietary ("fglrx") drivers working with Beryl, but apparently, the latest release of Beryl has something that is incompatible with the ATI driver. After about 4 attempts at making this Edgy-fglrx-XGL-beryl combination work, I decided to give the open-source "radeon" driver a try with the AIGLX extension to X.org.
And to my surprise, it works! I'm typing out this entry in Ubuntu Edgy with Beryl working with AIGLX! And, unlike an issue with the fglrx driver, I can safely shutdown the system without it locking up occasionally when switching screen modes! Awesome!
Granted, the performance of the open-source "radeon" driver is not as good as the proprietary "fglrx" driver, but considering that I didn't even have to go outside of the Ubuntu repository and the Beryl repository to get his working is quite amazing. And it's a quick configuration change if I have to drop down into the normal "2D" metacity desktop.
I'll keep working on this over time. Looks like I'll continue using the open-source driver barring any major issues. In the meantime, I'm going to create another image snapshot of this configuration to go back to..