Welcome aboard! Good to see the tri folks back. I've been away myself for a while.
Great advice - go cheap. Other than a bike, helmet and goggles, you don't really need anything special. A cheap bike works. I agree the knobby tire mountain bike, although fine, is not ideally suited

I like Craig's list and you can always find an older bike out there. Actually ask some of your friends, I am sure there are a few that have bikes that they do not use. Go to garage sales, etc.
Tri shorts and tops are nice but you don't need them. Just use a swim suit.
I started tri's 18 months ago after my last Goofy, just got tired of only running. Multiple sprints, couple of Olympics, 3 Half Ironman races and doing my first full Ironman in 5 weeks, IMFL. Just wanted to warn you, it is addictive.
I forgot who mentioned the WDW 70.3 in May but rumor has it that it is not going to be at WDW in 2012. Check out the site, the course maps are not up and registration is not open. That is highly unusual for a Iron branded race.
enjoy,
Duane