I'm just wrapping up my undergrad now, so looking back, here is what I would recommend.
1) Buy the computer right before school starts. Technology gets obsolete fast, so buy it as late as you can (you really won't benefit from a laptop in the summer anyway). Besides, you'll probably get a free iPod if you buy in Sept. (on top of the edu discount).
2) I would suggest the MB instead of the MBP. Given your typical needs, the MB is more than enough (word, movies, a few games, etc.) and it's much more cost effective. The new models (Oct. 14?) will probably be quite nice, and from your parents perspective, this will be a 'compromise' (they may actually pay for it). A HUGE point is weight! The MB is lighter than the MBP and, from my experience, it is designed perfectly for the college/university student. That is, unless you're doing hardcore video/photo editing, the MBP is overkill.
3) Laptops, although convenient, are in no way necessary for college/university. A desktop would easily do and there are always library computers. If you want a laptop, go ahead and spoil yourself (I did), but don't feel that you have to. The iMac is just as good and will probably last you much longer. There is also the possibility of buying an iMac and a cheap netbook for portability - just something to think about.
Finally, SAVE as much money as you can. Eventually, you'll move out of residence and be responsible for RENT and GROCERIES. Once you factor in the cost of beer, college/university life is really quite expensive.
P.S. Work your ass off for scholarships, then spend the money on booze. I did, and I have absolutely no regrets! Work hard, party hard!