I am planning a mid-April ride on the GAP as part a PGH-PHL tour. Note that there is one tunnel that typically does not open until April 10th, and the web site says there is no easy was around it.
Phil has predicted an early spring. Let's hope he's right this year.
DCwom: I believe Google Maps is incorrect. The interactive map for the Heritage Trail shows it crossing the Mon via the Smithfield St. bridge:
http://map.friendsoftheriverfront.org/