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Like it or not, I can hear everything they're saying.
If this is honestly true, I don't think you have anything to be embarrassed about--but I still think that it's a bad play to mention you heard the conversation; there's too many ways for Bob to take that the wrong way, even if it was unavoidable.
One thing that comes to mind, after very little thought, is that you can catch up with Bob (phone, e-mail, bumping into him at the cafeteria, whatever) and mention your upcoming possible promotion (which you know he knows about) and ask him whether you did well on the project where you worked for him. Mention that you want him to be honest and that you are personally unsure how well you did--which apparently is untrue, but try to draw out the answer he gave your boss.
If he says you did well, and specifically says you didn't do anything wrong, AND you're sure he wasn't kidding with your boss, then you have my permission to kill Bob in his sleep. I HAAATE people who lie to others about their performance to make them feel better.
Also, kick ass at the things that Bob said you did badly, of course.