I am a nonresident alien and am deciding whether to find jointly with my US citizen husband for 2007. Medicare and Social Security Tax have not been withheld last year.

IRS section titled "Nonresident Spouse Treated as a Resident" states that: "You may still be treated as a nonresident alien for the purpose of withholding Social Security and Medicare tax".

My concern is whether I would be liable to somehow pay Medicare and Social Security Tax for 2007 if I jointly file. I would also like to know how/if I can change the filing status right before filing the taxes.

Thank you for your help in advance!

According to the IRS website, you will NOT be held liable for Social Security and Medicare taxes even if you file jointly with your spouse.

That is a change of position by the IRS from past years.