Well, first of all, reballing a BGA phone processor is not an easy task, but it can be done... maybe not with just an hot air gun, but with a reballing station or a preheater and a hot air station, and some expertise it isn't impossible.

I would try the repair a broken phone, but changing an IC it in a good one just to see if it works (and even worse, in a N900) has no sense...

Anyway, it doesn't seem obvious to assure it can or it can't be done... let's just say no one is willing to risk their N900...

Oh! and that the simplest way to know if a newer uP is a drop in replacement isn't to read all the datasheet!! you only have to ask the TI support! they already know the answer to that question!