i use to be in the same exact position as you Darth, had a mountain bike and had some road slicks on it. then i was going to put road gearing on it. but i'm glad i just went onto craigslist and bought a used roadbike for $200 (03 khs flite 500.) best decision i ever made. for me it cost less to buy a used road bike than to convert my mountain bike to be more road worthy. my average went from 15-17 mph to 18-20 mph. also i thought you had to baby the road wheels, thinking that it was fragile, but i was wrong. i ride in chicago where there are many potholes, and sometimes i jump off curbs from sidewalks sometimes. the wheels are still true. plus my bike felt wayyyyyyy lighter. so carrying it upstairs/downstairs, or over obstacles makes it pretty easy.

mountain bikes are great for short commutes as someone stated above. but if you were to ride longer than a few miles, road bikes are best. the wheels are bigger, skinnier, has better gearing, better geometry, in other words, road bikes is more efficient in pedalling than mountain bike.

last year my friends and i did the mcdonalds night ride (around a 25 mile bike ride around chicago), and my friends that had road bikes beat me by 20 minutes. the first couple of miles i could keep up with them, i could even hit 30 mph easily on a flat road while my friends on road bikes couldn't. but after a few more miles, i couldn't keep up with them anymore. thats where the efficiency of the road bikes came in. my other friends on mountain bikes and i were too tired to keep up with my other friends on their road bikes. although it mostly depends on the rider on how fast they bike, having a road bike makes your pedalling more efficient, so you'll pedal less to get the same distance as you would have on a mountain bike.

so in short, i think you should go for a road bike. buy a used modern road bike on craigslist, its way cheaper than buying a brand new one. police auctions are a good place to buy bikes too.