Hakkinen retired from F1 back in 2001, but has recently been spotted testing with McLaren's driver simulator at its Woking base. A Spanish newspaper is also reporting that Hakkinen underwent a physical exam to evaluate his readiness for the sport, at the request of McLaren’s Ron Dennis.
The chief executive, Martin Whitmarsh, previously claimed that “they currently have no plans to run Mika alongside Alonso next year,” and that “the identity of our second driver will be announced in due course." If Hakkinen does end up with McLaren, he will be joined by Pedro de la Rosa and Lewis Hamilton as well as Alonso for the 2007 line-up.
Update: Looks like the rumors were false. McLaren has announced Lewis Hamilton will be the second driver for 2007.