George Clooney Trades Stacy Keibler for First Lady ... For a Night

George Clooney's girlfriend definitely has her priorities -- she missed a night at the White House for a modeling shoot, but George found a suitable sub -- Michelle Obama.

George was at 1600 Pennsylvania Ave. for a White House State Dinner, honoring British Prime Minister David Cameron. Other famous attendees included Vogue editor-in-chief Anna Wintour, producer Harvey Weinstein, and actor Idris Elba.

As for Stacy ... we're told she left last night for Barcelona for a modeling shoot.

So we gotta ask ...