Way back in December 2005, a message was posted to Apple’s darwin-dev list asking for help porting x.org 7.x to Mac OS X. I was on holidays at the time and, having some free time, albeit temporarily, volunteered.
By June 2006, we had a working copy, working in the sense that it all built and started up. not working in the sense of actually, well, working. I could launch an xterm, and it would start with completely black windows and, if I recall correctly, no working mouse or keyboard. It was kind of hard to keep up with a fast-develping upstream source when I could only devote a day or two at odd intervals. The source would refuse to build even though that module had built for me the month before and so on. It was a pain.
Kevin Van Vechten (Apple BSD Group manager) put Ben Byer on the job last fall and things really started moving. Apple fixed the major issues, the black windows, non-functional mice and keyboards, even the terrible startup times and then pushed the source changes upstream. The announcement was greeted with great fanfare and celebration. Okay, there seems to have been no reaction whatever, a tad disappointing. 
There are some questions of library compatibility, the newer x.org libraries have a higher compatibility_version than the older Apple X11 libraries. This means that programs linked to the older libraries can still run with the newer ones, but that programs linked to the new libraries can not run if you remove the new X and reinstall Apple’s X11 from tiger. Should not be a problem for most people. If you go against all reccommendations and have DYLD_* environment variables set, you’re on your own. Anyway the issue got me to hack this up last night. A library version number editor. It was a fun hack, I strongly advise that you never use it though, there is a better way!