It's not really about Bill Gates, I've heard it about others before, but it's possessed of a certain dark humour that I rather appreciate and so I'll repeat it here...
Bill Gates is in the airport and is recognized by an old classmate of his...introducing himself: "Bill, don't you remember...it's me, Bob, I was a year behind you...we had the same math teacher..." and poor Bill, he doesn't remember either him or any of the stories he's telling but he doesn't want to appear rude...
Bob continues..."Say, Bill, I'm going on vacation with this girl, just been dating her a bit, and you know, wouldn't it be swell if you came up and said hi when we're talking? I mean, you've done well and all and she'd be impressed..." and Bill, ever the sport, agrees, and Bob goes back to his guest.
After a short amount of time Bill approaches the couple, they're deep in conversation, Bill waits a moment and then interrupts..."Bob? Bob? Is that you? I can hardly believe it..." to which Bob turns and curtly responds "Fuck off Bill! Can't you see I'm talking to someone?"