The new Mac Mini is a roomy little bugger. Inside its aluminum body, there's an extra SATA connector and room for a second hard drive. You'll need to find a SATA cable, but iFixit is on the hunt for one.
And there's even more upgrades you can do. You can easily replace the existing hard drive and bump up the RAM. Sadly, the CPU is soldered to the motherboard and not user-upgradeable.
But that's OK. You can still soup up that $600 Mac mini for less than the cost of the $1000 server model. [iFixit]