The “definitive” answer to the current CA, MA, and NM same sex marriages will be provided by the IRS. Will these same sex marriages receive the married filing joint tax rate?
Not currently. Under the Defense of Marriage Act, In U.S. law, marriage is defined as being between a man and a woman, and the federal government won’t recognize marriages that aren’t.
Strictly speaking, it’s not any law that defines marriage, but the very definition of marriage dictates that it is something between a man and a women. So the correct question is whether the IRS recognizes domestic partnerships or the terminology to which your locale adheres.
Strictly speaking, Captain Amazing got it exactly right and you got it exactly wrong. For purposes of the OP the relevant question is the federal definition of marriage. DOMA is what defines marriage on the federal level. Domestic partnerships have nothing to do with the question. Marriage doesn’t have some mystical “very definition.” Marriage is what the law says it is. In future please refrain from offering answers purported to be factual when you have no idea what you’re talking about.
Jer and I are filing jointly this year, now that we’re common-law. His tuition and education amounts etc. are big huge deductions for me. 
Oh, but we live in Canada. 
Do you and Jer get the tax benefit in Canada versus opposite sex married couples?
I should have said, “Oh, but we’re Canadian.” (Jeremy became a citizen here a year ago.) Same-sex and opposite-sex couples who have lived together for at least a year in a relationship are considered common-law spouses for tax purposes, among other things.
What this means is that since Jer, as a student, didn’t make enough to be taxed last year (all student loans and bursaries, which aren’t even taxable AFAIK, and he doesn’t even have to claim the bursaries), he can transfer all of his deductions to me as his spouse. He’ll still file a return, but just as a formality to get him on the national voters’ list and to apply for the Goods and Services Tax/Harmonized Sales Tax quarterly credit. (I make too much to qualify for the GST/HST credit.)
For me, this means that I can claim his tuition and “education amount” (a monthly amount based on number of months as a full-time student) - as well as a couple of other things, I forget - on my own tax return. Deductions where before I had none. So where I might usually end up owing about $100 (CDN) in taxes, I’ll likely get a refund.
We won’t know until Thursday, when I get my T4 slip (Canadian equivalent of the form that lists your income, tax paid, unemployment contributions, etc.) and then sit down and do the return. It should be interesting.