Ok, I did this for the longest time. I had a hardtail mtn bike when I first started riding. You can put city tires on it on your own, any bike shop will have them. The tires aren't THAT thin but they're better than riding on knobbies if you're doing street riding.

You will have weight and gear challenges but I could sometimes average 16-17mph for centuries. To be honest, it wasn't *that* bad and sometimes I miss the fact that it felt like a tank and I could go over anything.

When I did the tri's last year, I saw all sorts of bikes. I passed someone on a Cervelo and I got passed by someone on a mtn bike. I think you can play with your bike enough to make it functional and then if 'n you want a road bike, go for it.