If you are flying from NY to Boston and plan to drive, you might want to reconsider and fly directly to Nantucket. USAirways has service from LGA. Space on the ferry is hard to get and relatively expensive and there are already too many cars on the island. Also, you don't really need a car.

If the wedding is in town, the White Elephant or the Jared Coffin House would be potential choices. If the wedding is in 'Sconset, the Summer House or the Inn at Waiwinet are great. There are plenty of other inns and guest houses. I would not recommend the Nantucket Inn at the airport. Visit www.nantucket.net for tons of info (and a great webcam of the harbor with sound).

Whatever you do, you should book soon because even though you are going after Labor Day your first choices will probably be gone if you wait.