I am working on my OPT, my F1 visa still valid and I file as a resident alien. I started work March 2007, but am still enrolled full time in the University till May 2007. Will my employer with hold FICA even though I am enrolled this semester ?

If you arrived in the U.S. in 2003 or later, then, for 2007, your F-1 visa exemption is still valid.

Your employer should NOT be withholding FICA taxes. Ask him to stop and refund what has already been withheld.