I was in H4 as a dependent for 4 years and changed to F1 Dec '2005 and got the OPT from Feb07 to Feb'08. I've started working on OPT(using SSN) and changed to H1 in Oct '07. What is my tax status? I am confused whether to file my taxes as a Non-Resident (based on OPT) or Resident (jointly with spouse). But for all previous 5yrs, I was filing jointly with spouse using ITIN. Now my employer didn't deduct Medicare/Social for the period I was in OPT. If I file the same way as I was filing before (resident) do I need to pay medicare/social that is not withheld in my pay cheque? Or Am I eligible to file as non-resident based on the F1/OPT VISA Status?

You are a non-resident alien for 2007 if you CHOOSE to be.
You would have to revoke your resident alien choice from the previous years.

Otherwise, you are a resident alien due to acrryover from your 206 tax return when you filed jointly wth your spouse.

If you retain your resident alien status, then, yes, you MUST pay Social Security and Medicare taxes.

Definitely I can not file next year(2008) as a non-resident, Is there any implications, If I CHOOSE to be a non-resident for 2007 to file a tax(as per F1/OPT)?

You have to revoke your choice to file as a resident alien IN WRITING by attaching the revocation to your non-resident alien tax return.

Once the choice is revoked, you CANNOT EVER file as a resident alien because you are the spouse of a resident alien.

In other words, if you file as a resident alien in 2008, it must be because you would qualify to file as a resident alien on your own, as if you were not married to a resident alien.

See IRS Pub 519, page 10 and 11, for details.