It should be no surprise if you follow the Phoronix benchmarks closely, but as shown by this World of Padman ioquake3-based test at 1920 x 1080, Ubuntu 11.04 with its open-source R600g driver is over twice as fast as found in Ubuntu 10.10. This is largely due to the Natty DRM code carrying Radeon page-flipping support and within Mesa the R600 default driver now being the Gallium3D solution rather than classic Mesa. Of course, the open-source driver is still multiple times slower than the Catalyst driver for R600/700/Evergreen/Northern-Islands is, but it is much faster than from just a few months ago. The Radeon X1000 (R500) speed is becoming much closer to the Catalyst driver speeds from 2009 (when the R300/400/500 support was discontinued from the Catalyst driver) at 70~80% the speed.
The OpenArena tests also show great performance improvements for open-source ATI Radeon graphics thanks to the latest DRM and switching to Gallium3D. The open-source Radeon stack can be made even faster if disabling SwapbuffersWait and enabling color tiling for R600+, which can be done from the xorg.conf and is illustrated in this article, but that is not done by default.
The Qfusion-powered Warsow game was faster in Ubuntu 11.04 than found by default in Ubuntu 10.10, but not multiple times faster like was experienced with the ioquake3 titles. The Warsow performance is more demanding than World of Padman and OpenArena, but the improvements found in Ubuntu 11.04 are enough to take this game from being barely playable to around 40 FPS with the Radeon HD 4670 graphics card.