JetBlue is currently running $39 fares starting July 4 through November 2, 2011. I am also seeing fares of $49 through December, including the Thanksgiving holiday time period. I went through the entire calendar and there are no date restrictions (7 days of the week are included). I only saw 2 or 3 days that do not have the low fares available.
The Terms and Conditions state that fares must be booked by June 20 (today) and 14 days in advance. They also state the fares are good through end of August, however, looks as though they go far beyond this. Round trip for less than $100! This is much less expensive than the train.
I am assuming these fares will not last long. Also, with JetBlue they will offer you a full refund if cancelled within 4 hours of booking.