That's a tough situation... if it were me:
A) I'd ask the department or other students how (un)usual that type of offer is, and what the chances are that you'd end up with an RA or something by the end of the academic year. If you only had to pay for one year, that's significantly less to deal with.
B ) Ask the school if there was anything they could do for you regarding housing/funding. During a discussion with the head of the department at one of the schools that accepted me (PhD, Eng), I was told that I should contact him if money was going to come between me and an acceptance. In this case, I was granted a 'partial' stipend (~9 months), and would likely be picked up in a lab for the summer to work. The offer was several thousand less than competing universities, which I told him.
Or it could mean:
C) You were effectively rejected. If they don't want you enough to pay, is it worth it?
D) You need to find a professor to commit to you before you get an offer. At two schools, I was accepted but needed to find funding (meaning, find a professor to commit to me) before anything else would happen. They made that pretty obvious though.
Hopefully you find yourself in one of the first two situations... Good luck!