The plans to switch over to Nouveau with kernel mode-setting support for Ubuntu 10.04 were great, but then in December there was the very surprising move that the Nouveau driver got pulled into the Linux 2.6.33 kernel. This complicated matters for Ubuntu as they already were set with using the Linux 2.6.32 kernel for the 10.04 LTS "Lucid Lynx" release, so they simply couldn't update their kernel against the 2.6.33 series to obtain this Nouveau support for "free" within the mainline kernel.
The mainlining of the Nouveau DRM code within the Linux kernel now provides several options for Ubuntu kernel developers and that has been to either back-port the Nouveau DRM from the 2.6.33 kernel branch, utilize an older snapshot of Nouveau's DRM back when the 2.6.32 kernel was their development focus, or build out the Nouveau support in a separate package as a back-ported module. Obviously the most ideal path would be to pull in the 2.6.33 kernel DRM since that's the most up-to-date and complete code available, which should provide the best user-experience and will also be more properly maintained in this Ubuntu Long Term Support release.
Ubuntu 10.04 Alpha 2 didn't carry any Nouveau kernel support, but it looks like the said kernel support will be on target for Ubuntu 10.04 Alpha 3. Not only will it be there, but it looks like the Canonical kernel developers have decided to pull the Nouveau DRM from the 2.6.33-rc4 kernel, which is good news for those wanting to use the Nouveau driver.
There was an Ubuntu kernel team meeting today and in regards to their kernel-lucid-kms blueprints, the Nouveau KMS code will be pulled in for the next alpha release. An Ubuntu kernel bearing the Nouveau DRM support can currently be found in Steve Conklin's PPA. The xf86-video-nouveau DDX to handle the Nouveau kernel mode-setting also isn't in the Lucid Lynx repository at this time, but hopefully it will hit there soon after the kernel is in place.
Ubuntu 10.04 Alpha 3 is scheduled to be released on the 25th of February while the official release of Ubuntu 10.04 "Lucid Lynx" is expected to arrive on the 29th of April. For those wanting to try out the Nouveau driver in an easy-to-use manner right now, the full stack can be found on Fedora 12 (along with other recent releases) where it's being used by default. Nouveau support for other distributions can be found in some package repositories, while more vendors should switch over to this reverse-engineered driver once deploying the Linux 2.6.33+ kernel.