Whyever would you want one?
You can get a server in the cloud for a fraction of the cost of buying and maintaining a physical box.
If you do have a vital need for a server on a short leash bandwidth and latency wise, you can get an Intel box for a few hundred bucks and put Ubuntu on it. (Mine has 4G, mirrored 1T disks, an admittedly trailing edge AMD64 dualcore, but cost in the $500 range - the case, motherboard and processor came for free).
I guess the only people who use these are design shops where there are only Apple skills, and they want something impressive to show people. (I've never seen an Apple server, but assume it's something like a titanium cube suspended in a tank of Fluorinert, glowing with Cherenkov radiation from the plutonium backup battery).