I visited the Four Cuisine bistro on Preston Street for dinner tonight and enjoyed it thoroughly. While there was only one vegetarian main course available, it was a very pleasant curry with saffron rice. The non-vegetarian options were appealing and varied, with an especially broad selection of seafood options. Also recommended are the chevre stuffed peppers, cheesecake, and Arrogant Frog Ribet Red Shiraz. The restaurant is elegant, but not stuffy or pretentious.
The number of customers seems to vary widely by night, so reservations on Friday and Saturday nights are recommended. Given how it is located just a few blocks from where Bluesfest, it might complement a day of concert-going nicely.
Full disclosure: a good friend of mine works as a server at the bistro.