From racing-live.com :

The prospect of Fernando Alonso moving to BMW-Sauber at the end of 2007 has stepped up a notch. Both the Hinwil-based team's boss, and current BMW racer Nick Heidfeld's manager, have refused to rule out the scenario when speaking to the Cologne newspaper Express. With Alonso's relationship with McLaren seemingly at a new low after the Hungarian race, speculation has named both Renault and rising Grand Prix power BMW as possible destinations for the disaffected Spaniard. "Since our fusion with Sauber, we have increasingly become a more interesting place for other top drivers," BMW-Sauber principal Theissen acknowledged. He added: "For me, the question is not whether we can offer the world champion enough, but in which configuration we can best make the next step ahead." It has been widely reported that, presently without a valid contract for 2008, Germany's Nick Heidfeld was the endangered current driver for BMW-Sauber. Express, however, suggests that the 30-year-old has now inked a lucrative deal for next year, leaving Robert Kubica more likely to exit if Alonso does join the team. Heidfeld's manager Werner Heinz seems open to the idea of pairing his charge alongside twice drivers' title winner Alonso in 2008 and beyond. "Why shouldn't a Heidfeld drive with an Alonso?" he wondered. "With the performances Nick delivers at the moment, I would have no worries about that." D.B. © CAPSIS International Source GMM

Well, as much as Nick is maturing and making progress and as much as Alonso is having a low, I don't believe that Heidfeld is a match for Alonso. It would be a typical first drive / second driver scenario