A ton of people have done exactly what you are doing,, and with good results. My wife and I have used mountain bikes set up with slick or semi slick for years. They make great commuters, around town grocery-getters, rail trail, all purpose bikes. I have to give somewhat of a warning though. Some people love the 26x1.5 tires and some hate them! I hated them! A few years ago I purchased a set of the Kenda Kwest 26x1.5 and it was one of the worst tires I have ever used in my opinion. If you are a lightweight, you may be okay, in that you can run them at a low enough pressure to be comfortable but I weighed about 175 at the time and tried them at a range of pressures and was really dissapointed. They are a fast tire, but they are also harsh and I had a real problem with traction. I ended up throwing them away after sliding the rear wheel out turning at a low speed and crashing.

However, If you do not like the Kenda Kwest, don't give up! There are many other tires out there with some more volume that are excellent. At the top of my list, and many other peoples, is the Schwalbe Big Apple. I like the 26x2.35 size. These are not a cheap tire, but are well worth the price. Every bike I have put Big Apples on has been transformed! It really amazes me how much of a difference these tires make. Especially on an aluminum framed mountain bike.
Good luck, and if you have any more questions, ask away.
Bruce