I wanted to do surgery and got surgery first. While it's a little more intimidating that way, you'll have a huge advantage in terms of enthusiasm. At least, the way it worked out with the other people on the rotation (many of whom were doing surgery first because they had no interest in it), was they got miserable, fast. They were able to cover it for a few weeks and seem somewhat enthusiastic about that 2 a.m. rectal exam, but by week 5 it all wore kinda thin. Not only was it affecting this particular rotation, it was making them think all of third year was going to be like that. Meanwhile, I was having a blast; third year was *great* as far as I was concerned. I was learning so darn much, from people I liked being around (I guess the future surgeons like being around surgeons). And I learned a lot more because people appreciated the enthusiasm.
So, while I know it's intimidating, just hang in there for the first couple of weeks (your head will spin), let people know you want to do medicine and that you're really excited to be there. [Don't complain that you didn't want to do it first].
Best of luck. You're going to have a blast.
Anka