"What IBM gets out of it is simple - a much faster processor of their own."
And this isn't trivial. Despite the good point that Haddock made about IBM having excess fab capacity, I still don't see how IBM can deliver a 90nm 3GHz PPC970 without AMD's help. In addition, as yourbankruptcy points out, what about Power5? Or even their mainframe chips? Not that performance is all that important for mainframes..
I am still not sure how IBM would do this without taking at least an equity stake in AMD, but AMD does have some stuff to bring to the table. Except for cash, I don't see how they can do that.