If you follow the Ubuntu install it should work with the following tips:
-be sure to install libpci-dev
-run ./configure --prefix=/usr
-make, then checkinstall will create a deb package for you and install it.
The second step is the most crucial, otherwise X won't be able to find the driver. This was all done under Debian Lenny AMD64, Radeon Mobility X1400. Driver works great for basic stuff.