Join Date: Feb 2006
Location: Long Island, NY and Boca Raton, FL
Programs: JetBlue TrueBlue, AAdvantage, Rapid Rewards, Sky Miles, SPG, Marriott Rewards, HHonors, Hertz
Posts: 2,275
They will probably implement this feature for TrueBlue members soon. In the meantime, as mentioned above call 1-800-JETBLUE and they'll help you out with seat availability.
FYI: In recent months, B6 began holding back 'prime' seats from advance bookings. These are the exit rows and aisle seats in front of aircraft. However, on MOST (ie. not-as-full) flights, you can grab one of these great seats if you check in online 24 hours prior, as they are released at that point.