While Ubuntu 8.04 Alpha 1 won't ship with the open-source RadeonHD driver by default, it's easily obtainable through the Ubuntu Universe Repository. In Ubuntu 7.10 there is even the RadeonHD driver in Gutsy, but it's quite out of date.
Everybody who knows the Debian experimental repository could create his own backports. It's no problem at all to use pbuilder with a simple 3d driver. I would not overrate this issue - universe has basically >90% simple backports from sid or experimental. Basically you only need to change the debian/control file:
xserver-xorg-dev (>= 2:1.4)
xserver-xorg-dev (>= 2:1.3)
that's all! You really need 8.04 for a 1 char change