I don't want to sound disrespectful, Satoshi (after all, you're the reason we're all here!), but I'm not sure that "it's probably good enough" is a sufficient answer!

I hear you saying that SHA-256 isn't much better than SHA-128, but what I think we need to hear is why SHA-128 is BETTER than SHA-256...

(Also: Is there a planned mechanism in place to switch to a new hashing scheme?  Seems like a good thing to plan for early, even if it seems unlikely to be necessary...)