After considering things a little more, I can't see Ahmadinejad stepping down if he is forced to dismiss Mashai. It would bolster the forces that have been protesting the election, and Khamenei sure as hell doesn't want that.

But if Khamenei wants Mashai gone from the post, he'll be gone. The simple fact that the hard-liners are against Mashai means that even if he did stay in office, he'd have no real power (even considering that Khamenei makes all the decisions anyway). Hell, Ahmadinejad's the same way. Khamenei says jump, Ahmadinejad says "How high?"

By Monday, at the latest, Mashai will be out. Hell, I'll be surprised if he lasts to the weekend.