My H1B 6yrs period ends by May 2012 and I would like to transfer my H1B to a new employer.

1. whathappens if I join based on transfer receipt but previous company revokes my approved I-140 while the transfer is still pending? Will i lose my status? Will USCIS deny my H1B transfer based on revoke I-140? Also, is it safer to join only after it gets approved?

2. Can my current Priority date be maintained when applying for I-140 through new employer? what happens to the PD if I140 revoked? Can I file I485 with my new employer even if the I140 with previous employer is revoked.