I am married to a US citizen, but I am not a resident in the US. We understand we can file jointly for our taxes if I get an ITIN. Can we also back-date this application and claim this joint status for previous years? We married during 2007.

Yes you can file joint return for 2009. Attach W-7 (ITIN application) with your tax return.
When you get ITIN, file amended tax returns for 2007 and 2008.
Both of you must report worldwide income on the tax returns. Your U.S. Tax Return: Filing Status for Married