I also came across the same situation last year when I started working under OPT status. I did some research on the IRS site as to what kind of taxes I had to pay. What I found was that, on OPT, you are only liable for state and federal taxes i.e. you dont have to pay Social Security and Medicare.  Check the following link
http://www.irs.gov/businesses/small/international/article/0,,id=106287,00.html
There is more info available on the website. You may have to spend some time to find it. I took all the supporting documents to my employer and they refunded whatever SS and Medicare they had deducted and also stopped deducting them from my future paychecks.

Hope this helps.