Never say never. Froyo currently runs (in very crude form; drivers need work, as does power management) on an iPhone 3G today, dual-booting iPhone OS. (I'm currently experimenting with Android 1.6 on my 3G.) As long as the phone can be jailbroken, you can run another OS on it.

(For me, it's a perfect marriage. Apple makes incredible hardware; full-stop. Android is a vastly better operating system and developer ecosystem. The only unfortunate part is AT&T, but at least T-Mobile is available as an option for unlockers.)