It's really a tough call and definitely depends on how strict your interpretation of vegetarian is. I would not recommend the event to a vegan.
Items are plated in front of you and the chefs are all present so you can easily ask about ingredients (easily, that is, if the station isn't slamming) or ask for a modified plating (leave off the animal).
Menus change weekly so you never really know what's going to be offered.
Note: I'm not a vegetarian, although I generally don't eat mammals. It's primarily for health reasons, though, so I'm not concerned with hidden ingredients.
I'm currently looking at my booklet from the first PFTS last year (9/30/06) to give an example. Zones are color-coded to aid in navigation.
Red:
Artist's Point's Edamame Salad that was plated with the sesame crusted tuna was probably veg-friendly.
Jiko's Peppadew & Edamame Hummus with Vegetable Salad and Pita Chips were definitely vegetarian.
Blue:
Le Cellier's Celery Root & Apple Slaw that was plated w/the duck confit was probably veg-friendly; Roquefort Potatoes plated w/the beef may have been, too.
Garden Grill's Grilled Autumn Vegetables with Roasted Pepper Quinoa and Balsamic Glaze was definitely vegetarian.
Purple:
a couple of desserts that may have been veg-friendly, although I don't know about gelatin
Yellow:
Jean-Robert at Pigall's had a medley of mushrooms w/polenta that was plated with duck, but knowing he's a French chef, I'm thinking the mushrooms probably had some kind of animal-based stock in them.
Chef Selz had a white truffle polenta that was plated with a salmon. accompanying sauces were asian in nature so may have had fish sauce.
Green:
50's PT had Butternut Squash Tortellini, Chai Tea Wild Mushroom Ragu w/Cranberry Cream -- vegetarian.
American Restaurant had a succotash salad plated with a scallop, but there's bacon mentioned and I don't remember if it was in the succotash or just with the scallop.
3 desserts, but i don't know about gelatin.
Pink:
Spoodles had a spicy black bean relish and avocado creme fraiche that was plated with pork -- probably veg-friendly.
There are also cheese tables, but although there were people there to carve the cheese, I don't know if they were familiar enough with the particular cheeses to know about the rennet sources.