It seems fairly nailed on, but there are no sources for info. The only thing out there that I've seen is Button saying he's happy with his decision, the tone of which suggests he's moving on.
Still, he's leaving the sport on a high, ability wise. While it's hard to tell in something so reliable, he's not been embarrassed by Alonso as so many others have. He's still a strong driver in my opinion, it's just a shame this season has been so miserable.
He'll be a top endurance driver and after that he'll have countless offers for a commentary role should he wish to head down that road. I hope he doesn't just become a pundit though - he's got far more to offer in the box.
Top Gear? Hmm, not sure - and I've not seen anything other than the apparently baseless rumours weeks back. They could do with a figurehead star to go alongside Evans, but he doesn't fit the existing format, so we could be sure it would be evolving rather than trying to follow on from the Clarkson era (which is a good thing, as it would be an utter flop if they just try to be the same).