Gary:
IMHO, since you have been in country for over five years, the IRS will probably take the position that your exempt status has expired and that you are therefore liable to pay Social Security and Medicare taxes. This fact is reinforced if you filed Forms 1040/1040A/1040EZ as a resident alien for the 2004 tax season.
You can download IRS Pub 519 from
www.irs.gov to read up on your tax status. On or about page 46, they outline how to request a refund of Social Security and Medicare taxes if you feel you are eligible.