Adding my voice to those saying "If you can't afford it, don't buy it."
Coming from experience here, don't start down the path of relying on credit. You're young, bad things are most likely going to happen to you financially at some point. Don't build up unnecessary debt. It *will* bite you in the ass eventually, and that stigma can follow you for years, even after you get things cleaned up.
However, you most likely *do* need a laptop for college.
Two real options, both an order of magnitude less expensive than the rMBP:
1) Macbook Air - The Air is cheap, fast, and reliable. It's also very *light*. You can get an Air for less than half the cost of the rMBP, which will eat college work for breakfast, get damn good battery life, and won't break your back. Won't play games as well, but you can pick up a 360/PS3 for that (or get your roommate to foot that particular bill, in exchange for a few beers ;P )
2) Some low-mid range Windows laptop - Cheaper still, probably better specs, and you'd be able to play PC games on it. It'll almost certainly be heavier, have worse battery life, and of course, it won't be a Mac. But you'll save some cash. If you choose to go this route, stay away from Dell if you can - horrible build quality. I've had better luck with HPs at work, where we order them in bulk, although there's a bit of a gap between their consumer line and the business models we order.
Whatever you choose, consider getting some insurance on it. College is hell on gadgets, and when (not if!) something bad happens, you'll want to know you can replace it with a minimum of hassle.