You can't work without the OPT card in hand. I also had the same problem. USCIS took exactly 3 months to make a decision on my case (regular, no RFE or anything). Unfortunately my requested starting date was 2 weeks earlier than the decision date. I thought USCIS would make adjustment on the starting date, but they didn't. So I lost 2 weeks of my OPT time. This was in January 2010.

A friend of mine also had the same problem this fall, but USCIS somehow adjusted his starting date. I don't understand how USCIS operates.