NOM HAS RESPONDED TO THIS NEWS HERE.

Here is an excerpt from the Attorney General's letter:

The President has also concluded that Section 3 of DOMA, as applied to legally married same-sex couples, fails to meet that standard and is therefore unconstitutional. Given that conclusion, the President has instructed the Department not to defend the statute in such cases. [source]

UPDATE: First reactions to Obama's decision to stop defending DOMA

UPDATE 2: Maggie has a few initial thoughts here