Without anything more than the decree, the IRS might be able to come after you as well. The IRS may provide a form that clarifies your obligation if both you and your soon to be ex spouse sign off - I'm not sure.
Have you searched the IRS website?
This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship. This attorney is licensed in Texas.
If you're married for any part of 2011, then you're still married for 2011. You can still file married, filing singly, but you might be better off filing maaried/jointly. Check with a CPA who can can run both sets of numbers to see which is better for you.
This may also affect your divorce settlement, so before you do anything, this is something to check with your family lawyer, as well as your CPA.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
You tax filing status is determined by your marital status on December 31 of any given year. If you are legally married on that day than you must file as a married person either individually or jointly. Filing individually will offer you some protect from her failure to file.