The Nouveau stack in Ubuntu 10.04 LTS provides 2D and X-Video acceleration with kernel mode-setting. There is no 3D support / OpenGL acceleration within Lucid's Nouveau stack, but it's being worked on and will be flipped on now in a future release. There are, however, Ubuntu developers working on creating a PPA (Personal Package Archive) of the Gallium3D Nouveau driver for interested users. Just this week we delivered benchmarks of Nouveau's Gallium3D driver atop Fedora 13 where it's now available.
For those interested in proper OpenGL support for gaming or other situations, the NVIDIA binary driver can still be installed in Ubuntu 10.04.
The announcement of finally flipping on the Nouveau support in Ubuntu can be read on the ubuntu-x mailing list.