OP, I know exactly how you feel, because I'm going through something similar. (My story is below.) I would advise you to hold onto your acceptance for now, but retake the MCAT, ideally before June if you think you can do well (could you manage April or May?). It won't affect the acceptance you already have, but it might help you get off waitlists at schools you like better. If that effort fails, though, I think it would be wise to attend the school that's accepted you.
My problem was that I took the MCAT twice in quick succession and got 7's in the PS section both times, due to very bad nerves. I did well in the other sections, though (13 verbal and 11 bio). I'm married with kids, so I really wanted to go to med school where I live (NYC area), but I knew I was doomed with that 7. I thought really hard about it and talked to my husband, and decided that if I had no other choice but to leave home for med school, that's what I would have to do. So I added a bunch of faraway schools to my app which might be friendlier to someone with my MCAT score. I also put in a huge effort with all the schools to write really good secondary essays, and sent them all supplementary essays and several rounds of LOIs as well. And I planned from the beginning to take the MCAT one more time, in January (the first opportunity in 2009). I was sure I could improve my PS score, and I thought there was a small chance I could use a better MCAT to snag a late interview somewhere, or at least help me get off waitlists if I landed on them.
As it turned out, I did great with the out-of-town schools and horribly with the NY schools. I got interviews at 6 out of 12 out-of-town schools, but absolutely none in NY. I got my first (and so far only) acceptance in Feb--but instead of making me happy, it did the opposite, because the school is 800 miles away. The thought of going that far away from my family really felt awful to even think about, even though I really and truly want to go to med school. I put down the deposit, but found myself praying that something better would come through. In the meantime, I got waitlisted at a couple of OOS schools which are much closer to my home.
About 10 days ago, I did get my new MCAT score, and it was a huge improvement: 12 PS-11 VR-10 BS. I immediately contacted all the schools where I haven't been rejected yet, as well as the WL schools. I found that the WL schools were very interested to hear about my new score (although nothing has happened with either of them yet), but schools where I haven't interviewed flatly refused to even look at it. This was very depressing, because my hope all along was that I could get an interview in or near NY. But as a last-ditch measure, I sent a personal appeal to the dean of admissions at the one NYC school that hasn't rejected me, which also happens to be my dream school. To my amazement, I actually got an interview there, on the next-to-last day of the whole season. I should be hearing from this school in a couple of weeks.
So, although you certainly can't count on a new MCAT score helping you in this admissions cycle, there's a very small chance that it might. HOWEVER, you must be very sure that you're ready before you take the test again, because rushing into it and getting a bad score won't help you at all.
