My Spouse has meet the requirement for the "first year choice" to file as a resident for 2011. I have stayed for only 20 days in Dec 2011. Can we file a joint return as resident? Or I need to file a 1040NR only?

Yes, you can file joint return as residents as "First Year Choice."

In fact, you do not even have to be preent in the United States to file jointly with your spouse. You could be on Mars for all the IRS cares, just as long as you have either a SSN or an Individual Tax Iendetification Number (ITIN) (for hwhich you can submit Form W-7 to have the IRS issue you an ITIN).