Using a Mountain Bike on the road.

No one will hate you, except the people on fancy road bikes that you pass during the race (yes I have seen it happen).

Easiest way to make a MTB road-friendly is to put on some slick tires, something like this.
(be sure to purchase new tubes of the appropriate size as well)

This entire upgrade can be done for about $30.

ditto.

I just completed my 3rd tri on a mountain bike yesterday.  two sprints and an oly.  not only has nobody said anything negative to me, nobody has said ANYTHING at all to me about it.  Nobody cares .    The only thing I've found folks care about is that you stay to the right if you aren't passing someone.

As far as rules go, the only one i think I've ever heard regarding bikes is you need bar end plugs.  And a helmet, of course.

Like bryank, I've logged alot of training miles on the mountain steed and i've been able to be fairly competitive on it by:

1) putting on 1" slick tires.  really skinny ones.  big difference

2) clipless pedals and tri shoes

3) i tightend up my Judy Rock Shox (circa 1995) suspension fork as much as it can go.

4) got a cycling computer to work on cadence

of course I want a sweet tri bike, but my wife wants a 2nd floor bathroom more, so i'll have to wait.  In the meantime I'm working on the engine and looking around for exactly what I want so i'll be ready to pull the trigger if i see a good deal.

good luck.  ride proud.

I did 2 sprints and an Oly on my 38lb Trek 4500 before I moved up to a RB and TB.  You can sign up for the Fat-tire division if the event has this category, most do.

Tires and a new rear cassette really helped me.  I got a 12-23 to replace the original 11-32 and with 8 speeds I was easily able to get the right gearing for the road to maintain proper cadence.  If you don't have one, get a computer with cadence such as the Cateye Astrale 8.

I did my first tri on my MTB - put slicks on it first.  I passed a few road bikes too.

The most important thing is to do it and have fun in the process.

Another Giant MTB user here!  I'm doing my second full tri this weekend with it - it's a little humbling, but I figure I'm just paying my dues.  I recently brought it in to my LBS to have my rear derailer fixed and the owner asked me what I averaged on speed........when I told him about 18 to 19 mph he looked sort of impressed and said I was doing well with it considering (I felt a little better after that).  I have a Trek 2100 in my sights for next year as my first road bike (tax return giftie to myself).

Wow that is fast!  I thought I was doin well hitting the 14's on my mtn bike!

I didn't know you could trade it in, I'll have to ask him......although I had thought I would keep it in case I ever wanted to take it "off road".  It's had kind of an easy life - sort of like having an SUV that's never seen any dirt :-)

I certainly don't feel very fast.  Just this past Saturday I was chugging away up a steep hill, certain that the bike was turning to lead on the way up, when I was passed rather quickly by a guy on a road bike that called out a cheery "Good Morning!".   I'm not sure how my smile/grimace of pain was interpreted.....but he sped up after that.