Upon further talking with our in-house veterinarian staff, I wanted to let you know that Bearded Dragons will sometimes stand with their mouths open to let out excess body heat. There is not always a need for concern if your Beardie is doing this. Of course, if it does alarm you in any way we encourage that you take him in to see your vet, because it is always best to err on the side of caution.
Although this is an article placed in our kid's section, I thought maybe you would like to take a look at it... it is our Bearded Dragon Facts, which explain some of a Bearded Dragon's common actions. I learned quite a bit from it myself: http://www.drsfostersmith.com/pic/a...
As always, we hope that you seek the advice of your veterinarian if your Beardie is doing anything that might seem out of the ordinary for him.