I am pretty confident DOMA section 3 (the only part that actually does anything) will get struck down next year, and the definition will revert to the states. Before Scalia's off-the-wall dissent in Arizona v. United States I thought it would be unanimous on 10th Amendment grounds. Now I think maybe 7-1 or 8-1 depending on whether Kagan recuses.

I think there is virtually no chance that Thomas will vote against a 10th Amendment argument. The only hope congress has on this (though kudos for them for defending the doomed law) is to hope the court follows Comstock and gives the federal government broad authority to tack on all kinds of things to federal powers.

I think the chance of this though is very, very small.

However, if they do strike it down on 10th Amendment grounds expect a lot of hand-wringing from liberal law school professors. When a district court found it violated the 10th Amendment, Jack Balkin (very much on the liberal side) said something to the effect that while he wanted to see DOMA struck down, the 10th Amendment was just too dangerous to the idea of centralized government to allow that to stand.

Just to clarify, there were other dissents in Arizona v. US, but the only one that was bonkers was Scalia's. The other dissents (Thomas, and Alito's partial dissent) thought the AZ immigration laws were similar enough to the federal law to survive pre-emption. Scalia thought the state had inherent rights as a separate sovereign to keep out non-citizens it didn't want there. It wasn't perfectly clear why Scalia didn't think Arizona could keep out foreign tourists if they wanted (though he did assert that the state would have to respect federal visas). I even knew people who usually agreed with Scalia who thought that opinion was "radical."