Another question - could people use the DOMA decision as away to challenge laws in states where gay marriage is legal.
I mean, could a couple sue saying the law violates their Fifth amendment rights and use the DOMA ruling as some kind of precedent that it was overturned on a federal level?
Based on what I've heard of the DOMA decision, I'd say yes, but not directly. The Supreme Court placed a heightened scrutiny standard for laws that discriminate based on sexual orientation. Which means that you need a better reason to discriminate on that basis than to discriminate on (most) other bases. So if I were arguing to overturn a state ban on marriage equality, I'd argue the hell out of that decision.
I may be able to explain the Prop 8 decision. Standing is one of those concepts that's easy to state but often tricky to apply in a particular case. Standing is basically "why do you care?" Taken to an absurdity, it means you can't sue if I get into a car accident with my next door neighbor. One of the principles of standing is that you can't challenge a law simply because you're a taxpayer who doesn't want your taxes spent on X, Y, or whatever.