I'm a resident alien that got married in 2007 but my spouse lives in another country. I've continued filing as single because we were married in another country. Can I amend my past returns? Can I now file as married filing jointly? My spouse does not work. Would this affect future immigration efforts?

Yes, you can file joint return as residents. You must both declare worldwide income. Attach W-7 (ITIN application) with your tax return.

You should file joint return for 2009 and after you get ITIN, you can amend your past returns.

Not that you can amend back to 2006 (and the 2006 amendment MUST be file no later than 15 April 2010).

Any tax return for 2005 or earlier CANNOT be amended for a refund due to the three-year statute of limitation for claiming refunds.