Have you seen the new '08 Treks yet? I'm not sure if the shops have them in yet, but here are some web shots.
The Soho is really nice. The 1.0 is 700$
http://www.trekbikes.com/us/en/bikes...n/soho/soho10/
Theres a single speed version too(I love this model):
http://www.trekbikes.com/us/en/bikes...an/soho/sohos/
Both have mounting points on the front and rear. So your cargo should be taken care of.
And kinda bypassing all this info too, you could just try to find an old road frame somewhere and restore it for your budget. That would also give you the benefit of learning how to work on your bike. I find that if you learn how to do repairs and fixes early, you're more likely to stick with riding as your first transportation option instead of having to check back into "autoholics anonymous".
Also mind the fact (like gregtheripper said) that you will need to save some part of your budget for light(s), bag, tubes, rack, and other accessories.