There are lots of people who are working, or have worked, on money for GMing. Ryan Dancey ran an organized play for a while. There's Otherworld Excursions (
http://www.behemoth3.com/otherworld/), who have run at least one tour. heck, shortly after 3e came out some players contacted Onte cook and asked if they could pay him to DM for them. (I don't know if he'd have done it anyway, but my memory is they were in a different state, and Monte just gave them a realistic idea what it would cost to be worthwhile to him, and that was the end of the matter.)
Since an important part of GMing is coming up with neat ideas, any freelancer is in part getting paid to act like a GM, though I admit that's not all of it. But I often take things I originally developed for home games and make money off them, especially as pdf products or some of the lower-paying magazine articles.
I have had professionals in other fields offer to pay for my room, board and travel expenses to come GM for them, and I've even done it a time or two to meet new people and make contacts. (Having an author you like ask if you're free to come hang out is always exciting.)
I've even had casual friends ask if they could pay me to GM, but I've never seen those as really serious offers. I have a limited amont of time, and can't game with everyone I'd like to. I doubt anyone who has the resources to would -really- be willing to truly make it worth my while to professionally GM. To allow it to take up professional time (instead of leisure time), I'd need professional money. Besides, if I had a good time and got to like those players, I'd eventually feel bad taking money to run games for them, but wouldn't have the spare time to run games otherwise.
I do wonder if electronic gaming is going to change some of these dynamics. If I didn't have to leave home, and things such as miniatures and maps were easily taken care of for little expense, it might become much more economical to pay me to GM. And some sort of style guide and rating system might allow players to find quality GMs who run the kinds of campaigns they enjoy.