This move may have something to do with President Obama's recent "evolution" on the question. And some recent polling from the Pew Forum on Religion and Public Life points to an additional set of data that may help explain why the NAACP could take its stance. Black Protestants (mostly evangelical) are nowhere near as set in their opposition to gay marriage as white evangelicals. Over twice as high a percentage (33 percent) approve as do white believers with similar theological underpinnings (14 percent).
Gay marriage is coming.
The NAACP announcement reminded me of the first announcement from a national civil rights organization in support of marriage equality that I know of. In 1994, before I'd much thought about gay marriage, I remember running across a notice of a pronouncement from the Japanese American Citizens League.
That's getting out in front on civil rights!