I found exactly the same problem with the live CD (ATI Radeon 9200, Athlon64, Edgy x86_64) but decided I'd try installing via the alternate CD's text mode anyway and saw exactly the same again. I've managed to get in to recovery mode via grub and changing the graphics driver to use 'vesa', or installing and switching to 'fglrx' works fine. As far as I can see this points pretty strongly to the 'ati' driver currently not working with a bunch of Radeon cards out in the wild.
-- Install 6:10 Failure: Black screen at Live Desktop Stage https://launchpad.net/bugs/67487 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
