The other day at practice we got to discussing jammers jumping forwards over the line right as the jam whistle went. Some people said they had been caught out doing this and got a penalty. The relevant rule is 4.4.2:
Jammers may not be accelerating at the jammer start whistle. They are, however, permitted to be moving, coasting, or braking
There was some discussion on this over at zebrahuddle, and the consensus seems to be that if you jump forwards and are fully airborne as the whistle goes, you are not accelerating (as unless you have a jet pack on, there is no way you can be gaining momentum midair). But if you are still touching the ground (e.g. with your back foot as you propel yourself forward over the line) you can be deemed to be accelerating.
It might depend on the ref, but I would say that if you want to try this tactic you will need to be very careful with the timing! And I would always play it safe - wherever there seems to be a "strict" reading of the rules that is the interpretation we should go with. It is not up to the refs to give us the benefit of the doubt.
Hope that clarifies it for people.