Well, it's a sad day here in Washington. Stephen Colbert's portrait has been taken down from where it hung "in between the 2nd floor bathrooms and within view of the 'America’s Presidents' exhibition," and left the National Portrait Gallery. However! It has moved instead to the “Treasures of American History” exhibition, currently displayed at the National Air and Space Museum, as the American History Museum is still undergoing renovations--the place where he was trying to get it hung in the first place!
The National Portrait Gallery blog has all the details. If you live in or around Washington, I highly recommend checking out their blog (suitably named "Face to Face") on a regular basis!