Excellent guide thank you. Have Mountain Lion working perfectly on a 2008 XServe2,1. The only major issue i had was the ATI X1300 graphics card on it. Even without the drivers, the machine performed fine with obvious exceptions like slow Launchpad, but Remote Desktop was beyond sluggish. Copied across the X1300 drivers from Lion, and all is fine 🙂 Though i did have to remove the AMDSupport.kext from Mountain Lion as it was conflicting with the Lion ATISupport.kext that i'd put in. If anyone has any difficulties when copying drivers across, do look at Console.app, it often tells you what's causing driver problems.
Remote Desktop is still a bit slow, but useable. If that's all i have to put up with to make Mountain Lion Server work and keep a £3000 server running for another 2 years, then i'm happy!