There's a post on this down on the South board. The list of sandwiches includes places from coast to coast, so everyone should check it out.
Interestingly, one of the more obscure items on the list, Iowa loose-meat sandwiches, are also available in Gainesville, Florida, at a place called Steamers. I ate a lot of them during my one year in Hogtown. The place is owned by an Iowa native.