The recommended way though to obtain the driver for now is using git, seeing as there is new work going into the xf86-video-radeonhd driver everyday. However, if git or compiling from source frightens you, just sudo apt-get install xserver-xorg-video-radeonhd or fire up Synaptic and look for the RadeonHD driver package. If you need help building and installing the driver on Ubuntu, check out our Ubuntu RadeonHD tutorial.
Hopefully come April the RadeonHD driver for ATI R500 and R600 graphics cards will be in a state to be included and enabled by default in Ubuntu 8.04 LTS. Back in September, Mark Shuttleworth talked to us about AMD's latest open-source kindness and expects to fully integrate their latest work for Hardy Heron. AMD's new AIGLX enabled driver will also be available through the Restricted Driver Manager in this next Ubuntu installment.