The mini can be used as an external hard drive by connecting it via a firewire cable and starting it up in Firewire Target Disk Mode. You do this by holding down the T key during the mini's startup, until it appears on the desktop of the host system as any other FW drive, and files can be moved or copied on or off it as with any other drive. Note that it can't be used this way as a means to drive another system, such as an older iMac, where the intention might be to use the old system's screen and keyboard.
It has to be said that this isn't really a good way of using a mini for prolonged periods - while it works, it's only really intended for file transfer. The mini is clearly a rather expensive way to add an external drive to another system!
Apple have a knowledgebase article explaining firewire target disk mode at
http://docs.info.apple.com/article.html?artnum=58583