RE: the presidents decision not to defend DOMA. I have the same concerns that Megan McArdle raises.

The Imperial Presidency
theatlantic.com

Maybe it was always thus, but it seems to me that both parties are increasingly resorting to procedural tricks rather than politics, and it worries me. Maybe this means that our political system is broken, maybe it means that the parties are getting increasingly ruthless—or maybe I’m overestimating the extent of the change. But as I say, it worries me. I think it would be disastrous on a whole lot of levels if the GOP managed to undo ObamaCare with this sort of thing. But if the precedent stands, I think you can expect them to try it the next time they have the presidency.

I think she’s right. What happens if HCR is still going through the courts and a Repub pres takes over and decides to use the same tactic and just declines to fight lawsuits. Even though I’m against both DOMA and HCR, I don’t know if this is an appropriate tactic to make changes in current law. OTOH, I found these 2 interesting pieces in the Volokh comment thread:

PRESIDENTIAL AUTHORITY TO DECLINE TO EXECUTE
UNCONSTITUTIONAL STATUTES - 1994
justice.gov

and….

Defending Congress - 2001
scholarship.law.georgetown.edu