I’m slightly surprised that the White House press office gave out this image, from her 1976 Princeton yearbook, in which she quotes Norman Thomas.
Norman Thomas was, of course, the leading American Socialist politician of the 20th century.
Perhaps they figured it would come out eventually so they wanted to be able to say, “That’s old news. We thought it was so inconsequential, we mentioned it ourselves.” Or perhaps they felt that substantively didn’t matter; Norman Thomas was a pretty mainstream and respected figure, as socialists went. And his quote is universal. Or perhaps they didn’t notice.

UPDATE: Gawker offers this theory: “or perhaps the White House planted it there knowing that it’d send the wingnuts into a hysterical frenzy, thereby distracting the sane wing of the party, what little there is left of it anyway, from forming any sort of cogent opposition to Sotomayor, making them all look like rabid fools in the process, just as they have each and every time they’ve tried to throw the socialist card on Obama in the past? Then again, maybe Obama’s just so confident in his nominee that he doesn’t care about trivial horseshit like her quoting a socialist over 30 years ago in her college yearbook?”