At DEN right now (connecting on to IAH) and wish to relay something I'm sure happens on occasion, but it's the first time I've witnessed it first hand:

Just as the door to our aircraft is about to close, the Captain (who has been personally greeting each passenger as they board) notices 3 empty First Class seats. He asks the FA to give him the manifest and says, "Let's move some good customers up front."

He selects 3 Premier Executives and instructs the FA to go back and invite them to move up and to "thank them for their business."

3 very surprised and pleased Premier Executives soon joined us.