The boys and girls over at iFixit have posted a 9-page tutorial on how to remove the Mac mini’s optical drive and hard drive and replace them both with bigger, faster hard drives.
This can only be done to a new Mac mini, as the older models use the older style PATA connectors for the optical drive, while the new one is all SATA.
The procedure looks easy enough, but does involve some technical knowledge and some soldering. And, without a doubt, this would void your AppleCare – but it is a really cool way to void it.