To all of the e-mails and forum posts we've come across in the past day, no, this public driver doesn't support X Server 1.5. Non-Ubuntu users will either need to wait for Catalyst 8.11 next month or install Catalyst 8.10 but then extract the fglrx 8.54.3 Ubuntu package and copy over the binary driver components.
In fact, the official release of Catalyst 8.10 doesn't have a whole lot worth talking about. It does have UVD Linux support enabled to accelerate video playback via the XvMC extension and their own HD video interface. However, all attempts at using XvMC with mplayer have failed thus far and AMD has been mum on how to actually use this feature. This looks like another feature we'll need to wait until Catalyst 8.11 to find out more.
It's been over 24 hours since the Catalyst 8.10 driver for Linux first hit the web and there are still no release notes. On the release notes page at the time of publishing is just "At time of posting this document was not yet ready. This file will be posted once it becomes available." The ATI Linux RSS driver feed mentions "Driver version: 8.542 NEW FEATURES: (1) AMDTM CAL: Computer Abstraction Layer runtime and libraries support. (2) Support for Red Hat Enterprise Linux 4.7 ISSUES RESOLVED: (1) X startup failure on Radeon X1xxx and later AGP cards. (2) Xserver crash after logging out multiple times. (3) Bad Xv video playback performance on AGP cards. (4) Artifacts left on the screen when quickly moving an Xv video window."
There is already quite a bit of feedback on the Catalyst 8.10 Linux driver and Ubuntu's special driver in the Phoronix Forums in a number of different threads.