How about this - Citysearch has a listing of restaurants supposed to be open on Thanksgiving:
Joe Allen is great, Big Pink is very casual but would probably be a fun thanksgiving, I've never done Blue Door but it's quite posh, being in the Delano and all, nor have I done Shula's Steakhouse.
Another possible way to get a lead on restaurants that may be open is to check on booking a reservation through Opentable.com. Not all local restaurants are on there, but several are. I just tried it and there are several that are apparently booking for Thanksgiving day. Worth noting:
Chef Allen - this is North Miami so geographically undesirable if you're fixated on South Beach, but I'll bet it's good. Here's the menu:
In South Beach:
Americana (in the Ritz Carlton
)Escopazzo (good Italian)
Mark's South Beach (one of my favorites)
The Forge (Mid-Beach, actually; a somewhat silly place but I'll bet it's good for Thanksgiving)
I'd check and make sure they're really open.
Well I've been out of Miami for about 3 years now, but there are a number of great restaurants I still frequent. So I don't know who will be offering traditional meals, but I'm sure most of them will.
Actually, I'm doing Thanksgiving in Miami this year (at an apartment, though) but I think we're bringing in Joe's Stone Crabs side dished to go with the turkey.