Hi - I was on a F1 visa from 2001 to April 2006. I then went home, and then returned in September 2006 on a different F1 for law school. I started working on OPT in October 2009. I changed over to H-B status October 1, 2010. Since 2007, I thought I was a resident for tax purposes and I have been filing the 1040EZ instead of the 1040NR. However, when I was working on my OPT, my employer did not withhold social security and medicare during the time I was working on OPT saying that I was on an F1 and not subject to social security & medicare taxes. Was this correct or am I a resident for tax purposes? Will I have to repay the medicare taxes they did not withhold?

I appreciate your assistance.

Yes, you are resident since 2007 and must file resident return. You should file your 2010 return as resident.
IRS will not contact your regarding FICA taxes. If in future, IRS contacts the employer, the employer may approach you to refund your portion of FICA taxes.