Jack "Ghost Rider" Sweeney at Bike Commuters wrote an excellent article regarding commuting on a retired mountain bike. Pun intended.
After a couple of weeks researching the 'net, I decided on the tire I really want. Specialized Nimbus Armadillo tires. At $45 a tire I just tripled what I have invested in the bike itself with the tire swap. The kevlar flat protection might be worth it though.
Another option is the Specialized Hemisphere. Of course, it has "Flak Jacket" protection rather than "Armadillo".
More to read here.