i agree that it seems unlikely, although there is intriguing circumstantial evidence which makes the story possible:
1) Schumacher and Ross Brawn loved working together, and would fall back into their working relationship immediately.
2) Brawn would be able to quickly advise his design team how to optimise the car for Schumacher's preferences, based on their long history together.
3) Schumacher was launched into Formula 1 by Mercedes, from his sports car days, driving for Mercedes when the team was run by Norbert Haug. Perfect circle and all...
4) Brawn/Mercedes have not shown any great interest in retaining Button's services. The prospect of having Schumacher in the team makes Button rather redundant, so they would not be too sad to see him at McLaren. In fact, they would probably prefer it to having Raikkonen there to fight against.
5) Kimi has announced he is taking a sabbatical for 2010. This suggests Button is a done deal at McLaren.
6) Barrichello can then claim he knew it was all going to happen, which is why he jumped ship to Williams. No one will believe him, but it will make him feel better about being dumped... :)
7) Much is made of the fact that Schumacher is still on the Ferrari payroll, but this has been at least partly because he retired at the top of his game (apparently against his will, depending on who you believe) and they were smart enough to keep him around and prevent him going elsewhere for the last 3 years. Now Schumacher is 40 and they have Alonso on-board, they are probably not so worried. He has already been relieved of his race-team position.
8) He was rather enthusiastic about jumping into Massa's car earlier this year, wasn't he? And Montezemolo was making noises about trying to run a third car for him, again probably in preference to having him go anywhere else.
9) Testing restrictions make every lap of testing priceless. And Schuey is the best test drive in the business, which would be invaluable to Mercedes.