Just 6.5 inches wide and 2 inches tall the square shaped box has a brushed metal finish with rounded edge it features a slot-loading CD-RW/DVD-ROM drive, USB 2.0, FireWire 400, DVI and VGA ports. It comes in two models, a 1.25GHz G4 with 40GB HDD system for $499 or an 1.42GHz G4 with 80GB HDD system for $599. Both Airport Extreme and Bluetooth are optional extras though, I'm surprised, I would have thought it'd ship with at least Bluetooth by default.
Apparently Steve Jobs described it during his keynote as a "Bring Your Own Display, Keyboard and Mouse" box. It'll work fine with with Apple's own peripherals, but you can also use other "industry-standard" peripherals.
So, a sub-$500 Mac, who'd have believed it...?
Update: The Mac mini is available in the UK for £339.00 for the 1.25GHz model or £398.99 for the 1.42Ghz model directly from the AppleStore, shipping as of Jaunary 29th, and adding an internal Bluetooth module will cost another £35.