I believe Massa will return to Formula 1 in 2010, if not in 2009 already. The rumoured (temporary) replacements for Massa, however, are:
Marc Gene
Michael Schumacher
Fernando Alonso
(Any driver who recently left F1, including Sebastien Bourdais, David Coulthard, Takuma Sato, Anthony Davidson) <-- This is what they did when Mika Salo replaced Michael Schumacher. They hired Salo instead of Ferrari's 3rd man Luca Badoer.