Committed (in this Git commit) to the xorg-server tree this afternoon is a bump to the video driver ABI (ABI_VIDEODRV_VERSION) after developers noticed the video drivers built against the ABI of X.Org Server 1.10 are now producing a segmentation fault when running the latest server code. Developers have tracked down the change causing the seg-fault and it's intentional.
While the open-source drivers are quick to adopt to major version changes of the X Server, it's with the proprietary drivers where it can be a problem, even with a small change such as this break to ABI version 11. Fortunately, at least on the NVIDIA side, they are generally quick to at least put out a beta or pre-release driver to support the new ABI version. In the case of AMD, however, that's where it usually takes a couple of months.
In fact, it's usually not until the next Ubuntu Linux release with the new xorg-server that AMD puts out a supportive driver of that kernel / xorg-server. Even then it's usually not on time to the point where AMD almost always has to pre-release Catalyst drivers to Canonical so their binary blob works in time.
In the case of Ubuntu 11.10, Canonical is planning to still ship X.Org Server 1.10 and not the forthcoming 1.11 series. This means there won't be this video ABI break to worry about this time around, but there may be kernel breaks to deal with as Ubuntu 11.10 will be shipping with the Linux 2.6.40 kernel or potentially even Linux 2.6.41.
Still, I'd not expect AMD to officially support X.Org Server 1.11 until at least this October or November. The upstream xorg-server 1.11 release is slated to occur in August. It would certainly be a pleasant surprise though to see AMD support the newest kernels and X Server releases in a more timely manner.