An INS memo issued January 29, 2001 discusses returning to the U.S. with an H-1B visa after an H-1B petition with a new employer has been filed.  CLICK HERE for the full text of that memo.  Essentially the memo explains that a person who has changed jobs can reenter the U.S. with "evidence that a new petition was filed timely with the Service Center."  The best such evidence is a filing receipt (form I-797), but the memo leaves open the possibility of other evidence if the filing receipt is not yet available (discuss the alternative evidence with your attorney before leaving the U.S.!).