Hi,
I entered US in the middle of 2005 on the F-1 visa.
During my OPT (Jan 1 to Sep. 30, 2008), my employer didn't withhold my Social Security Tax and Medicare.
My H-1B took effect since Oct. 1,2008, and SS/med has been withheld since then.
Now I plan to fill extension for 2008 tax return and claim resident alien for tax purpose for 2008, so I was wondering if I will be asked to pay Social Security Tax and Medicare during OPT (Jan 1 to Sep. 30, 2008).
Thanks very much!!

If you are single, then you can only file nonresident tax return or dual status tax return.
Only if you are married, then you can file joint as residents. No you do not pay FICA taxes on OPT income.
You must pay FICA taxes on H1-B income.
Read about H1-B tax filing: Your U.S. Tax Return: The U.S. Visas

Thanks very much! It's very helpful!

Another question: (I am going to fill married joint tax return, my wife: F-1)
In form 1040, line 39 (b): "If your spouse itemizes on a separate return or you were a dual-status alien, check here" (then use schedule A).
I was wondering if I can choose not ot check 39(b) and to claim standard deduction ($10,900) since I am going to fill extension and claim resident alien for entire 2008 year.
Thanks
M

Yes, you do not check 39b.

Thanks. It's very helpful