Urgent! xiaobaitu, radiology, or mytwocents helps please!

I am working in company A with OPT valid until May 2007. However, my h1B was approved and will be effective since Oct. 1 2006. But, another company B offered me aposition, and they strongly want me to get started working there in the beginning of September (next month). Since my h1B won't effective until Oct. 1, I am really concerned

1. if I can use my OPT to work for company B in coming September, and then transfer my H1B to company B after Oct. 1, or

2. transfer h1b now, and go to work now or wait until Oct. 1 to work for company B.

Thank you very much for help in advance.