The new Mac Mini is a roomy little bugger. Inside its aluminium 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 one 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 seriously soup up that $US600 Mac mini at a fraction of the cost of the $US1000 server model. [iFixit]