According to Bryce and the Ubuntu-X team, UXA will not (as of this moment) be enabled in Jaunty by default due to reports of instability.
To enable it, you'll need to add a small entry in your xorg.conf Device section under the identifier.
If you have enabled UXA, the team could really use your testimonials on performance and reports of any bugs you encounter. You can contact them in numerous ways with your reports:
Option "AccelMethod" "UXA"
Ubuntu-X mailing list
Ubuntu-X on IRC
Note: It doesn't matter if you are using the official packaged Intel driver (2.6.0) or the newer xorg-edgers PPA driver from Tormod Volden (2.6.99).