I have both a Surly LHT and a Trek 520 for touring. My trek is better for good road and med loads. However, I set my LHT up with a 44/32/22 crank and 26 x 45 tires so that it can handle rougher roads (eg dirt, loose dirt, gravel and poor roads like I see out west and in Europe). So I use it more like a hybrid mountain/touring bike. It has worked perfectly for me. The 26 inch wheels works very well on very poor roads.
These bikes may not be perfect for you like they are for me, but the biggest problem I found for a mountain bike frame for touring is the longer (common) top tube which I don't like on a touring bike. I do like the 26 inch wheels on mountain bikes for rough road touring. Unfortunately, most touring bikes in the US use 700c wheels, unless you look at some European touring bikes (which are very expensive).
A lot of people modify mountain bike frames for touring and they work out great if you choose the right component. However, I don't think a stock mountain bike, especially with suspension forks make a very good touring bike. If the geometry of a mountain frame fits you, then just switch out the components, change out the knobbies for high quality touring tires, and set it up for more upright touring posture.
Check out this link for info about touring bikes, mountain bikes and what you may want to a change out:
http://www.bicycle-touring-guide.com...-bicycles.html