i competed in my first tri (wrote about it here) on sunday on my cyclocross bike with road tires and it worked out great. a cyclocross bike is a little closer to a tri bike than a mountain bike, but you'll be fine. lots of people at the race i did had mountain bikes and hybrids and there were several racks on the back of bikes. don't worry about it. your main goal for your first tri should just be to have fun. that was mine, and it kept me from stressing about things and i had a great time and did better than i had hoped!