As long as INS received the petition for your H-1B visa transfer, you can starting working for the new employer. No need to wait for the approval. INS is currently processing this type cased filed in Sep. 2003. If you are not sure whether you really want to work for the new employer, you can wait and withraw the petition for transfer later before its approval. That way, your current H-1B visa will be still valid, and it will not affect you anything.
Runan Zhang
Attorney at Law
runanzhang.com
