Effective September 16, 2013, same-sex spouses will be treated the same as opposite-sex spouses for all federal income tax purposes.  This means that employers should no longer impute income to employees for the value of certain benefits provided to their same-sex spouses and that qualified retirement plans, such as 401(k) and defined benefit pension plans, must extend spousal benefits and protections to same-sex spouses.  For example, as of September 16, 2013, a qualified retirement plan must, among other things, provide same-sex spouses a qualified joint and survivor annuity payment option and treat a same-sex spouse as an employee’s default beneficiary, absent the same-sex spouse’s written consent to naming another beneficiary.

A copy of the relevant IRS guidance can be found here.