I'd put money on Massa staying at Ferrari through 2009. He won't get the axe for a couple mistakes. Remember he had rough form early in 2006 as well, but he turned it around to put in excellent performances in Turkey and Brazil.
He doesn't need to be as flawless as Kimi, because he's the de facto #2. Rubens had his off-days at Ferrari too remember, yet he made an ideal wingman for Michael. As long as he continues to qualify well and push the Mclarens down the grid away from the back of Kimi, he'll have a role at Ferrari. Of course if he drops it in the race six more times this year as he has done for the past two races, that may change. But I really doubt that will happen.
Besides, why would Ferrari bring in Alonso when Kimi is still there? Only if Kimi retired would it make sense to do so. Alonso and Räikkönen would not get on well as teammates and would derail development of the car because their driving styles are very different. Ferrari knows the most efficient way to win championships is to have a practical, if not official, #1 driver and focus the team's efforts on maximizing his points possibilities at every single race.