I am in the US for the entire year in 2008. From January 2008 till Sept 2008 I will be earning income on OPT Status. From Oct 2008 till Dec 2008, I will work on H1 B status (about 92 days). Will I have to pay social security and medicare for the 92 days period that I will be on H1 B status? Or, do I have to pay social security and medicare for the entire duration of my stay in the U.S. in 2008 including during the OPT status period?

Thank you in advance for your response.

Yes, FICA (Social Security and Medicare) taxes become effective the day your H-1B visa takes effect.

During your OPT period, you are exempt from FICA taxes.

On F1 and OPT, everyone is exempt from residency (and social security and medicare taxes) for 5-years. So if you have not completed 5-years, your OPT income is not subject to FICA taxes. Read Your U.S. Tax Return: Social Security and Medicare (FICA) Taxes for Non-resident Exempt Individual

Your H1-B income is subject to FICA taxes from day 1.