Any Constitutional overturn of DOMA would have to be based on an Equal Protection argument, not Full Faith and Credit, and in doing so would have to overturn not only DOMA but also all State bans on same-sex marriage, save perhaps that the "marriage"/"civil union" distinction might be maintained.
I've been thinking that if SCOTUS does deal with DOMA, it will be a narrow ruling that leaves the issue of interstate recognition of same-sex marriages for another time and focuses mainly on the federal-state recognition/benefits. I would be totally for them striking down all state bans but I don't see that happening right now.