Unfortunately, optical drives are not swappable on the mac mini. Just make sure you get one with the superdrive from he get-go, and you'll have everything you need. Blu-Ray drives won't be coming out for some time on the mini, so you'll just need to purchase an external when the need arises.
Drives are swappable, although it's a huge, HUGE pain (trust me, i'd worked as a mac tech before I went back to grad school this year). I'd recommend what a previous poster did and just purchase an external. Go to sites like macmall.com, newegg.com, or tigerdirect.com, and you'll be able to find several hard drives that match the form factor of the mac mini, and are stackable below the mini.
Just make sure you get 2 gigs of ram, you'll want it with the less potent core2 processor that they come with. Overall, they're great machines - the only reason I sold mine is due to the opportunity of acquiring a Mac Pro coming about - it's my baby