It's been in limbo for a while now whether Ubuntu 10.04 (the "Lucid Lynx") should ship with the xf86-video-intel 2.9 or 2.10 driver, but Bryce Harrington has announced this afternoon their decision. The Lucid Lynx will be running with the xf86-video-intel 2.9 driver. In addition, similar to what they are doing with selectively back-porting their DRM to run on an older kernel, they plan to back-port some of the patches from the 2.10 and 2.11 DDX releases back into their Intel 2.9 driver.
The decision to stick with this aging release is based on the fact that Intel stripped out user-space mode-setting (UMS) support from the xf86-video-intel 2.10 driver and that leaves this X.Org driver to only supporting kernel mode-setting. Kernel mode-setting is certainly superior to the old user-space mode-setting and KMS is where all the X.Org drivers are moving to, but there are a few caveats with Intel's support. At this time there's reports of some Ubuntu users with the very old Intel 8xx series IGPs running into KMS problems or quirks not being handled correctly with these newer code paths.
With Ubuntu 10.04 being a Long-Term Support (LTS) release, Canonical is just going to stick with Lucid Lynx being the last release where Intel user-space mode-setting is supported, albeit for most Intel users you will still be greeted by a KMS experience, just with an older driver and many back-ported patches. This decision was shared on the ubuntu-x mailing list.