Excellent, I hope we can get this in Chrome pretty soon too.
The big thing that IonMonkey is adding is type inference through static analysis, which is a big deal for JavaScript.
Crankshaft uses type feedback, not type inference. Ie: it will monitor the types of objects to generate type-specialized code, but will not do static type inference on the code to generate type-specialized code.
Chakra (IE9) already has it, I think.
http://blogs.msdn.com/b/ie/archive/2010/03/18/the-new-javasc...