iFixit has posted a fantastic, step-by-step tutorial tutorial explaining how to swap out the new Mac Mini's optical drive for a second hard drive.
Being the compact Mac Mini (and involving 2.5-inch laptop drives), you knew it would be a bit more complicated than your standard, tower bay component swap. The process does require a few moments of soldering and careful notation of various internal wires/sensors, but the tutorial has big, beautiful pictures to hold your hand every step of the way.
Also, to make things a bit easier, they're offering a 1TB, 5400RPM upgrade kit for $US250 that includes all the necessary tools and components. You could maybe do it cheaper, but that's how these guys pay the bills for these fancy tutorials. [iFixit]