Same sex legally married couples will now be allowed to file federal tax returns as married. The ruling takes effect on Sept 16th, so legally married couples on extensions for 2012 will be able to apply the new rules. Amended returns for all open years is also permitted. Of course, you’d only want to amend if it would benefit your tax situation such as if taxes would be saved or other benefits would be available only due to the MFJ status. State returns can only be amended and future ones filed jointly if the state recognizes same sex marriages, otherwise those individuals would still file as singe for state purposes. For futher information, see the IRS on the topic: Revenue Ruling 2013-17, or leave me a question on the blog.
On the MI return, same sex legally married couples will still have to file separately. MI has still not acted to acknowledge this status.