It is enough complicated question!
I am in the same boat and had to cancel my trip due to revocation of my not-expired visa by previous employer (he had not informed me about his actions and INS letter went to him).
From one side one can go to DoS site and read :
"Please note that you do not require a new visa merely because you have changed employers and received a new I-797 petition from the Immigration and Naturalization Service (INS). If your current visa is still valid, you may reenter the US with that visa as long as you are in possession of a valid I-797, even though the visa contains the information from your former employer." (
http://www.travel.state.gov/revals.html).
Or one can read at
http://www.murthy.com/arc_uds/UDh1pass.html the information about well-known Aytes\'s memo :
"The grey area which had been created by the INS and the U.S. State Department with respect to the validity of the visa stamp in the passport where an employee changes employers but has an unexpired visa stamp in the passport appears to have been clarified.
Commissioner Michael Aytes in a Memo in July 1997 stated that according to INS and Dept. of State policy, in the case of an H, L, O, or P nonimmigrant visa, the visa remains valid during its validity period regardless of a change in the beneficiary\'s employer.
The INS officer at the port of entry is supposed to stamp the I- 94 with the expiration date for the present employer on whose H1B Petition approval the Beneficiary is entering the U.S. although the visa stamp in the passport may have a different expiration date."
From other side, it is necessary to understand that in both cases it is mensioned the VALID and UN-EXPIRED visa. Valid - means NOT-REVOKED.
I am not lawyer, but this is opinion of our company lawyer ( no luck in this world...). The same is written at "
http://garychodorow.tripod.com/faqs/travel.html#Change of Employer by H, L, or O Nonimmigrants" :
"Change of Employer by H, L, or O Nonimmigrants:
An H, L, or O visa remains valid up until the date of its expiration. This is true even if the visa holder has changed employers since the visa was first issued. The procedure for entry is for the alien to present the new H, L, or O approval notice along with the old visa while applying for admission. As a practical matter, however, there is a risk in relying on the old visa because the old employer is required under INS regulations to notify INS if the foreign national leaves its employ, at which time the visa should be revoked.
References: INS CO Memo Michael Aytes, Asst. INS Comm\'r Adjudication, reprinted in 74 Interp. Rel. 1459 (9/22/97). Also see letter from H. Edward Odom to Martin L. Rothstein (2/12/97), reprinted in 74 Interp. Rel. 592 (4/7/97). ".
Of course, everyone can try to re-entry to USA with revoked non-expired visa (many people had done it before Sep 11th without problems), but in current situation I prefered to cancel my trip and I would be afraid to advice such way to somebody else...
).