X.Org Server 1.9.4 isn't too exciting as it just pulls in about two dozen bug-fixes. There's a couple fixes to EXA, DRI2, EDID, and RandR, but nothing too noteworthy. A bulk of the fixes are by Jeremy himself for XQuartz.
The XQuartz fixes shouldn't be much of a surprise as earlier xorg-server 1.9 point releases were also dominated by XQuartz fixes. Jeremy Huddleston is also an Apple employee and its the first time he has taken over xorg-server release management for the stable branch. It's expected the reasoning behind this is that with Mac OS X 10.7 "Lion", due out later this year, they will provide support for X.Org Server 1.9. With Mac OS X 10.6, their X Server support is still living in the 1.4 series.
The X.Org Server 1.9.4 release announcement can be found in this mailing list message.