It's not easy to say.
Bersani is looking at Grillo for a vote of confidence. This way there could be a semi-stable government (say for one year) and Partito Democratico would work on 3 or 4 points that Grillo has in its program (cut the number of parliament members, cut politicians' salaries, new electoral law, ecc).
If Grillo says he doesn't care and he will not vote the confidence, I really don't know what's gonna happen. I doubt Bersani will look for Berlusconi's support, it would be a political suicide for his party.
The President of the Republic will then be forced to try a government led by Berlusconi's party (candidate is Alfano). They will not have the confidence.
The President of the Republic will then be forced to try a government led by Grillo. Would Bersani vote the confidence? Possible. But I'm not sure Grillo wants this right now.
Bear in mind that the President of the Republic is in the final 6 months of his mandate, hence he cannot dissolve the Parliament, hence there is no chance of having new elections in the next few months.
At this point everybody is looking at Grillo and hoping he will cooperate. But his first statement was not promising, he said something like Bersani is a (politically) dead man talking. Even in the Movement 5 Stars plenty of people are asking Grillo to cooperate.
If he doesn't do it, my guess is that plenty of people will jump the ship.
But if Bersani then turns to Berlusconi for help and they fail (which is gonna happen pretty soon) Grillo will be stronger than ever, because people will have to believe what he's been praising for years, ie right or left make no difference, they're all thieves.
Sorry for my english, I don't know many techical words for politics. Hope it was clear anyway.