If you are thinking of transferring to a new employer please make sure that you know what is going on behind your back before it is too late. The best practice is to submit your resignation only after your new employer files your H-1B petition.
AILA has reported that it is the practice and policy of Vermont Service Center that if it receives a revocation letter from the current employer before it receives the transfer H-1B petition by a new employer, they deny the transfer petition because it is the VSC position that at the time the current employer revokes the current position, the H-1B alien immediately falls into a period of "unauthorized stay" in the U.S. This policy can push the transfer H-1B aliens into a tricky situation when the alien employees invoke AC 21 H-1B portability against the will of the current employer and the current employment contract requires 14-day advance resignation notice. It is thus imperative that the transfer H-1B employees should neither notify nor turn in the resignation before the new H-1B petition is "filed." Worse situation could be termination of H-1B employees by the current employer in a hostile environment. Since the current employer is likely to file the revocation (withdrawal) notice in the latter situation immediately, such H-1B alien can face a serious problem.