So let me be a mug: at a guess, they’re going to punt on Calfornia’s Prop 8 question via a technicality and shave as narrow a position as they can on DoMA. The Supreme Court is getting a little institutionally tired of being the heavy for every time Congress or a state passes a problematical law: see their responses to Obamacare and McCain-Feingold for examples of what they can do to provide negative feedback.
I am almost certainly wrong.
PS: I think that many commenters and commentators have a bad habit of over-emphasizing Chief Justice Roberts’ Obamacare opinion when forecasting future Supreme Court decisions. Although, to be fair, disappointment can often color analysis.