As strong as McLaren will be this year, I think Alonso is going to be the man to beat in the Ferrari, it's going to be a formidable combo.
Before Button joined McLaren, I would've agreed with you 100%. Alonso+Massa is a double championship winning combination.
The only problem for Ferrari is that McLaren went and got the most consistent driver in F1 (Button). By definition, the job of a No.2 driver is drive safely and consistently, bringing the car home in regular points finishes. When the lead driver goes out of the race, the No.2 driver steps up and is expected to finish very close to where the lead driver would've. Button is the very definition of the perfect No.2 driver.
Providing McLaren have a competitive car, I can't see how anybody can beat the combination of Button and Hamilton (though it should be close). If however, Ferrari rally behind Alonso and put all their resources behind him, then maybe, Ferrari will have a chance at the WDC.
In any case, 2010 is looking to be an absolutely fantastic season, with all the top men in top cars (unlike 2009, which allowed people like Mark Webber (no offence), to win a race and Fisichella, to almost win a race), while Alonso, Hamilton and Kimi struggled through a lot of the season.