I was going to go this year, but ran out of time / weekends / free time. I downloaded a book about fly fishing in the smokeys from Amazon.com, and I have also dealt with Smokey Mountain Anglers a few times. Nice guys. The don't like to come off information much, mainly because they like to guide ($$$), but spend a few dollars in their shop, chat it up for a while, and they'll put you in the right direction.
RushcreekAngler gave you a good link with Little River Outfitters. They have a website with a forum like this one that is primarily centered on fishing the Great Smoky Mountain National Park loaded with information. You can also download maps from this website denoting the various streams and areas to fish. I would go to this site first and or hire a guide as Matt Hougan recommended.
When I was down there, I purchased " The Fly Fisherman's Guide To The Great Smoky Mountains National Park", by H. Lea Lawrence. I actually bought the book at the Gatlinburg Visitor Center. The Park visitors centers didn't carry this book, but had another one, the title escapes me now. I am happy with this book though, and read it over the coarse of two evenings while there. It also has two different park maps in the back.
The park is actually pretty easy to fish, with plenty of pull offs for the upper Little Pigeon river (nothing more than a stream once your in the park) along Rt. 441 from the park boundary on up to around New Found Gap. Also, there are plenty of places to pull off for the Little River along Rt. 321. The Little River is one of the larger waters in the park, and frankly is downright dangerous in many places. These are all fast, high gradient waters. A wading staff is highly recommended.