Are you running Snow Leopard at 64 bits? If you run uname -v or uname -m does it say 386 or x64 (or the odd occasional 486 even)? x64 are the only ones running as fully 64 bit, which is mostly the latest Xserve’s. There have been some reports that you can boot holding down the 6 and 4 number keys to get into a 64 bit kernel but I haven’t been able to reproduce this in the various machinery running in my lab yet.
