http://www.autosport.com/news/report.php/id/63767
I know it sounds a little far fetched and quite honestly I cant see it happening really, but here me out since it's silly season.
The article only says he's retiring from racing in DTM, not from racing, this time last year there were some rumors Mika wanted to make a last run in Formula one and even went as far as to test the car in the winter, this was after being seen around the Mclaren garage at Brazil in 2006 and before Lewis was named the 2nd seat holder. However it came to nothing because Mika wanted to honor his dtm contract.
Now once again Mclaren has a free seat, and in reality the free agent market is very thin for top class drivers, right now Kovaleinen being the best option with only a season under his belt. If Renault dont get Alonso, they will snap him and Piquet Jr up quickly, leaving Mclaren with little to no options. Mika was once again spotted in Brazil, and has not long ago mentioned that he'd like to drive an F1 car again.
Like I said far fetched, but here is a world champ with current race experience and a good understanding of the team. He would be a good 1 year place holder until Nico Rosberg becomes avaliable.