Originally Posted by Josti
Carlos Reutemann drove as a guest driver for Audi and Peugeot in the mid-eightees, at his home rally in Argentina. Don't think he was seriously into rallying though.
Häkkinen drove a works Mitsubishi Lancer WRC at the Arctic Rally in 2003 (and maybe some other rounds too). This car was driven by Jasper van den Heuvel in some rounds of the 2007 Dutch championship (which he eventually crashed heavily).
As for Raikkönen, he will probably quit F1 after this or next year, if you believe recent reports. I think he will try rallying eventually, but I doubt he really wants to get as far as the WRC, let alone if he could.