This.
The best he'd be looking at is a role similar to Park. He's a quality player but he's not Manchester Utd quality nor would he fit into there style of play or formation.
I do agree it's unlikely that United would want him (for a start, didn't they introduce a "no player bought over 26" rule like TWO WEEKS AGO??).
But I disagree that he wouldn't fit in at United, simply because there is no such thing as a United "style". They played one system in the 90s, with the energy of Beckham/Scholes/Giggs/Butt in midfield. Beckham and Butt left, the rest aged, and they moved to a slower more patient build up, using a big strong target man/men. Then Van Horse Face left, and they changed their style again, to 3 quick interchanging fowards.
Two of them have now left - whose to say what style United will move to? History says that Ferguson won't swap like for like, but rather identify quality players and then work out a system to fit them. If Cahill did go to United, he'd be a reasonable success, because he's very good at what he does and Ferguson would find a way to use that.
Still won't happen though, because of his age if nothing else, but mainly because United will aim higher. However, United is a far better shout that the frankly crazy Arsenal rumours of last year - as if Wenger would spend big money on a older midfielder who can't pass.