While it may be good for Microsoft, Gates isn't coming back. He's pretty much committed to his foundation at this point and I think what he's doing there is far more important than what he could do at Microsoft.
Howard Schultz and Michael Dell come to mind first. Larry Page is the most recent.
Yahoo is a good counterpoint, though I'd suggest externalities like a lack of visionary leadership or a lack of intellectual capital, compared to their rivals, are more to blame.
Edit: Schultz wasn't a founder, but I believe he was their first visionary leader.