My campus/commuter/beater bike is a Giant Rincon that's like, 9 years old, maybe more. I've heard that back then Giant was using a road frame for some of their mountain bikes. I tend to agree (although it isn't light at all, look at the geometry), most mountain bikes have a top tube that slants down some. Mine is parallel to the ground, as as more road bikes. The rear section's geometry seems to be more compact than most mountain bikes. Here's a pic of a similar one:
I currently have a 26x1.75 mountain bike tire on the back, and a 26x1.50 Wal-Mart Slick on the front. It also has a Trek rack on the back (which has a solid platform so it doubles as a fender/splash guard) Its pretty comfortable and maneuverable IMO. I've ridden a bunch of my buddy's inexpensive full suspension bikes (NEXT brand, Mongoose, etc - WalMart specials basically). I don't like them at all. they are really soft for me, I feel like I'm riding a rubber band or something, flexing all over the place. LOL and the reason I've ridden them is because I was test-driving them after I repaired them because the people at WalMart suck at assembling bikes. One kid's bike was brand new and the brake levers would touch the handle and still be barely contacting the rim. another one, a Mongoose, the rear wheel was not tightened correctly. He went over a bump, and suddenly his back wheel is basically on like a 10 degree angle, rubbing the frame - I went to fix it and I easily took the nut off with my fingers. That's dangerous IMO and Walmart should be more careful...
/Rant