If you’re developing software that might get run on an Intel Xserve with Dual Xeons it might be useful to get one to test with. This Apple page about Intel build numbers show’s it’s using build 8N1215 of Mac OS X, which is (unsurprisingly) newer than all other builds.
Just to make things easier for those using Java, it uses build 1.5.0_06-113 and JVM build 1.5.0_06-68 (which is a JVM I don’t think ships on any other MacIntel boxes, and I’ve managed to crash :).