I am currently working with employer A on my OPT(expires on Aug 31st,11). Employer A's H1b petition is approved and h1b start date in Oct 1st,2011. I also got my cap gap I-20 which authorizes me to work for employer A.
I got a new offer from employer B and they wanted me to start working from Sep 1st,2011. Employer B's attorney told that I can start working for B once we get the H1b receipt notice from uscis.

1)employer B's attorney told that she will do a H1b transfer. How H1b transfer is possible when I havent started working on H1b(with current employer)? they should file new h1b petition right?
2)Say I accepted B's offer and started working with my receipt notice. What will be my status if B's petition is denied? Can I go back to employer A?
3)any risk involved in this process?