Epicurious has a great article on vegetarian mains from cookbook author Crescent Dragonwagon. (Thats her name. I shit you not.) Here's what Ms. Dragonwagon has to say:
"You need something visually big and bold enough to stand alongside—or in place of—the bird," she says. "I like to make a stuffed pumpkin. You can put it on a platter and let it be the visual star. It's as captivating and once-a-year-ish as the turkey." And though this dish might not go pound-for-pound against turkey for heartiness, it does take the place of another Thanksgiving favorite: the stuffing, which is probably soaked with turkey stock and off-limits for vegetarians. Moistened with vegetable stock, the pumpkin's apricot- and prune-accented filling is ready for everyone at the table to enjoy. And the pumpkin flesh itself, rubbed with salt, pepper, and brown sugar, is delicious.
The NY Times has a zillion recipes in their food section, and also quite a few veggie resouces. Here's Denise Landis on veggie thanksgivings:
One view of vegetarian dishes — an outdated one — is that they are too simple, and therefore lack elegance. Chefs like Mr. Tucker disprove this with dishes like the one he created last Thanksgiving: a rich mix of chestnuts, sage and corn bread roasted in squash and mounded with an apple-cider spiced ragout. There were also stuffed baby pumpkins and a roulade of wild mushrooms with seitan (wheat gluten) wrapped in pastry. The pastry was layered with yuba, a tofu product that has a crisp texture when roasted.
The Times Thanksgiving Special
And finally, here's an article written for carnivores about how to make Thanksgiving dinner more friendly for their vegetarian guests. I'll admit, its a little dumbed down. ("Start by sorting out what type of vegetarians you will be serving. Many won't eat red meat, poultry or seafood, but will eat eggs, cheese, milk and other dairy products. Vegans exclude all animal products, including honey.") But, it could be an easy thing to email to Mom or Dad in advance to give them some ideas.
Vegetarians Over for Dinner? Focus on the Sides.