How can client-side Javascript determine the iOS version of an iPhone?

I have a web site which can detect an iPhone using Javascript (navigator.userAgent.match(/iPhone/i)) but JPlayer doesn’t work with iOS3, only iOS4, so I’d like to tell iOS3 users why they can’t use the site, but let iOS4 users use it, instead of having the bug happen to iOS3 users.

I’ve Googled but haven’t seen a way to test for iOS3 vs iOS4. Does anyone happen to know of a way to do this in client-side Javascript?