Sheeri Cabral, a leading MySQL developer, wrote in The Pythian Group blog that, while he doesn't know if Widenius has resigned, "Some folks have known that Monty has not been happy in his current position; this leads me to believe the rumor is true." While I can't claim to know Widenius at all well, I had heard similar stories.
When I last saw Widenius, at the Linux Foundation Collaboration Summit in April at the University of Texas Super Computing Center in Austin, Texas, he didn't seem happy. Of course, trying to convince Linux leaders that Sun owning MySQL wouldn't mean trouble for Linux was like trying to talk mice into trusting a cat. Still, I got the impression that he wasn't happy with his new position at Sun.
But, for Widenius, suddenly a middle-sized fish in an ocean, I suspect it wasn't such a good deal. Oh, it made him rich for life, but it also left him without a professional challenge or an interesting job. So, why not leave?
Widenius can take it easy for a while, and, if he wants, he can start another company. In either case, he'll be his own man again, and I suspect that's what will really make him happy.