This release though is not exactly a surprise considering there's been release candidates for a few weeks and it was expected to make its debut in October so that it can be released as part of the X.Org 7.6 Katamari, but all has been quiet on that front, so it may be delayed or has just been held up until xorg-server 1.9.1.
Introduced in X.Org Server 1.9.1 are a few stability and performance fixes that have come since the release of X.Org Server 1.9.0 back in August. There's also some fixes to the XQuartz DDX and other areas.
The X.Org Server 1.9.1 release announcement can be read on xorg-announce.
When looking to take over management of the xorg-server 1.9 series, Jeremy had mentioned "[Apple is] probably going to pick 1.9.x as our next long-term support branch for XQuartz (replacing 1.4.x)." From this week's announcement of Mac OS X 10.7 "Lion" coming next summer, this is where we will likely see their X.Org Server 1.9 deployment.
On a related note, yes, we are looking forward to benchmarking Mac OS X 10.7 vs. Linux vs. FreeBSD vs. Windows 7 next year. There will be rock-solid support in the Phoronix Test Suite for Mac OS X 10.7 "Lion", but until it's released you can look over our Mac OS X vs. Linux OpenCL benchmarks, tests of Apple's enhanced OpenGL stack, and Mac OS X 10.6 vs. Windows 7 vs. Ubuntu 10.04 benchmarks.