Mountain Biking - Mountain bike good for the road?

I know it sounds like a ridiculous question, but can anyone give advice on a model that might be good on the road as well as decent on a trail?
I have a mountain bike and my wife would like to go with me on occassion, but she also would probably prefer to ride in the neighborhood more often simply for exercise. Since a road bike cannot be used in the mountains, but a mountain bike can be used on the road, and I cannot afford 2 bikes, then a mountain bike seems like the right choice. I'm just wondering if there are some mountain bikes that are better suited for mountain and road. I'm looking in the $550 range but would probably scan craigslist before I ever spent that much for a new bike. Thanks for anyones help.

my wife keeps one bike for riding on city streets and occassional trails/gravel around Toronto (we have in-town MTB trails in our ravines). She likes her rigid Miele Motion that she bought new at an auction for $190.00 cdn. Being rigid, less maintanance, very light, works on all terrain.

http://www.dirtworld.com/trails/trail.asp?id=865&trail_name=Don%20Valley%20Mountain%20Bike%20Trail

At the cottage she rides a Kona hardtail and a Santa Cruz Blur on the trails, and Trek and Opus flat bar hybrids on asphalt.

At your target price, Marin makes some nice hardtails, I'm not a Trek fan since they moved production off-shore, Specialized and Giant have some nice models...

...the important thing is that her bike be as good as yours. I've found that my wife and daughter enjoy riding more if we all have the same quality bikes...I also encourage them to lead on our familiy rides so that they set the pace (I lead from the rear)

Cheers

My first mountain bike 12 years ago was a Raleigh M50 no suspension, which I rode exclusively on-road. I left the knobbies on and went to town. I routinely did 20-28 mile rides, clocking an average of about 12-14 mph.

My current mtb is a Columbia Twin Peak, full suspension (sale price $329.00). I've only recently been learning to ride off-road (and love it :)). But I still ride on-road with knobbies. It was born a mtb with knobbies, and it will leave this world a mtb with knobbies. :D

When I ride on-road I've always inflated the tires to about 65-70 psi.

So, I think the point is that you can use just about anything for on-road riding... it depends on your strength and stamina. A road ride on knobbies it definitely a workout! But if she will only do easier trails off-road and ride primarily on-road, you could consider street tires.

At that price, I'd suggest sticking with a hardtail. Additionally, if it must be a do-it-all bike, you may want to look for a bike that has a lockout on the fork. It may provide some additional efficiency when peddling on the road.

If she's going to be riding local roads more, why not a hybrid? Decent on the road, and good for simple gravel and hardpacked trails. Otherwise, if she prefers mountain riding, try the above suggestions; or if she likes to kick it on asphalt, go for cyclocross.

look into the Gary Fisher Utopia.....I just bought one and it's a bonus 29er. Find a 07 model if you can....has way better parts. Mine has XT Crank, XT Derailers........

Thanks for everyone's suggestions. I will consider all of them, and I'm also going to have a nice, fun chat with a local bike shop.

i love my trek 4300, and use it daily as a commuter, even with the chunky tires on.