X.Org Server 1.9 RC3 doesn't bring much to the table, but it takes care of two issues that were moments too late to make it into RC2. The first change reverts an earlier commit to X.Org Server 1.9 that dropped mioverlay support on the basis of it being "unused", but the NVIDIA binary driver happens to use the mioverlay support (the open-source drivers no longer do), so this 2,000 lines of code was added back into the X Server for NVIDIA's benefit. Instead just the XAA overlay hooks will be dropped from the server, which is no longer used by any video drivers.
The other change is just Kristian Høgsberg signing off on a fix for a simple macro.
The release announcement for X.Org Server 1.9 RC3 (xorg-server 126.96.36.1993) can be found on the X.Org mailing list.