Jaunty will have the new Xserver 1.6, which brings a lot of 2D acceleration improvements and bug fixes but also a API/ABI change, so that binary only graphics drivers must be updated in order to work with it. This makes all currently released fglrx drivers (Catalyst 9.2) incompatible with Jaunty.
With the release of intrepid there already was a similar situatuation, but AMD released a special version for intrepid back then. But this time AMD just announced to drop the support for chips <= R600 (HDxxxx series) from fglrx coming with Catalyt 9.3 and recommends to use the open source -ati driver for the not longer supported cards. So the question is will Catalyst 9.3 support Xserver 1.6?
The good news is that the open source -ati driver in Jaunty will support all currently available ATI chipsets (yes this includes R600/R700), which will give you probably the fastest 2D acceleration among all linux drivers(intel just broke their driver by merging UXA) and also much better Xvideo accelereation in comparison to fglrx. (no tearing)
The back side is that there will be a regression regarding 3D acceleration. The open source 3D driver stack currently is much slower then the one in fglrx and the work to fix this will not be ready until Ubuntu 9.10 (or probably longer). This will not affect anyone just using Compiz – these will even get a major performance improvement in comparison with fglrx. But in games or more sophisticated 3D applications there will be a noticable slowdown.
And R600/ R700 chips will not have any 3D acceleration at all – not even Compiz. But for these there is still hope for Catalyst 9.5.