I am not a tax attorney or accountant, but here is my take:
If you're going to want the best possible return on your taxes, you'll probably want to file married jointly. This would be especially true if you are still living together. The downside is, you will probably have to split whatever money you get back with your spouse, assuming you don't arrange otherwise.
If you have been living apart, with separate finances, for most or all of 2011, then married filing separately would make more sense. It has the added benefit of being another one of those "truth darts" for your MLCer. However, your tax rate is going to be about the same as if you filed as single, so the amount of taxes you pay will go up; that will reduce your refund or increase the amount you owe.
(The above advice does not take into account things like owning a business, money earned from investments, rental properties, etc. At that point, hopefully you have an accountant who can help you make the best decision.)
Me: 45, Her: 40. Married 16 1/2 years, together(-ish) 20.
Status: BD 8/25/09, she moved out 8/28/10. No talk of D.
Every day is another chance to get it right.
http://www.vachss.com/mission/behavior.html"Counting days won't buy us years" —Wings by HAERTS
"Forgiveness means giving up all hope for a better past." —Lily Tomlin
"When we commit to our lovers, we implicitly promise to forgive them. There is no other way we can live with someone for better or worse or until death do us part." —Dr. Frederic Luskin