Using a MTB for Tri's?

At the shorter distances you'll see all kinds of bikes. If you have a flat course Tri you'll wish for more speed. See if you can borrow something from someone. The only other thing would be to try and use a road bike cassette on the rear (not sure if they are compatible) That would improve your speed ability by a few MPH. Absolutely use slicks for the ride, there are lots of choices, I have a set of Performance bike's slick city's on an old MTN bike that my wife sometimes commutes with. They roll much better.

If you can hold 20 MPH on a MTB you'll bike mid-pack. If you can borrow a road bike, you will probably pull off a top three or four bike split.

So it comes down to this, do you just want to finish or do you want to have a shot at age grouping? To finish, you'll do just fine with that kind of speed on a MTB, in fact you may surprise some folks.

Oh one last option, put a 48 tooth chain ring on the front! It won't shift from the middle well or maybe not at all, but you'll gain another MPH or two.

How about a road cassette?

Oh yeah, you will see lots of mtb at sprint distances.

Ok, thanks for the tips! Yeah, I know (believe) I can finish, but need to have something left for the run. If it's flat I think I can average close to 20 mph, but most likely around 18 mph so I can still run.
Thanks again to all!

Smaller gears on the back are essentially the same so no real help.

Weight weenie mountain bikers will often put a 9 speed Dura Ace cassette on a MTB to shave some grams though.

I used my MTB w/ slicks last year in my first sprint.  I was one of many in my race to use one.