IMWI 2010 use my tri bike. I'm not sure using a road bike would have made me any slower or faster.
I agree w/ Steve completely, tri bike without a doubt. I've been @ IMMOO 4 times and the hills are not as bad as the computrainer makes it seem. There are really only 4 hills on the course. This bike course is really about speed management where you want increase your effort and when to sit back and spin. The difficulty of the course lies in the fact that you cannot get into rhythm due to the endless turning and not from the climbing. You can stay in aero position for most hills and almost coast up half of them if ridden properly. Good luck at Moo!!!
Brian, you will be fine with a road bike. No worries. If all you want is to cross the finish line and hear the magical words ... a road bike is just fine.
I would say, just make sure the fit is great. If you can (and can justify the cost - I certainly could) I would get a Retul fit. Let the fitter know you are competing in Ironman and they will work to help get you fit for that. The less you can use your hamstrings on the bike, the better you run will be....it will be painful either way ;)