The only reason for browser-lockdown on iOS is that iOS does not allow third-party apps to interpret executable code (like JavaScript). This is because you could essentially create an AppStore within an AppStore and avoid review through delivery to the browser.
Most alternative browsers either don't support JavaScript, support JavaScript & Flash on the server side and deliver a "view" of the page, or use the iOS Web UI View control that is essentially an embedded version of Safari's engine and written by Apple.
I wonder if Google will use the Chrome browser for iOS to file a lawsuit against Apple for allowing alternative browsers much like Microsoft had to. Certainly Apple does not have a large enough user base in mobile phones to warrant regulation, but a couple more years of tablet dominance and the iPad could be a target for such a lawsuit.
Incidentally, Apple needs better support all around for "open in" type things. For example:
1) Open a URL in an alternative browser
2) Open an address in an alternative mapping app (like Navigon)
3) Open a calendar attachment in an alternative calendar
Gruber recently wrote an
article about iOS lacking some of the "Intents" and "Contracts" support found in Android and Windows Phone, respectively. He saw this as the next avenue for Apple to improve iOS -- I've been saying this for quite some time, so naturally I agree. The first time I noticed this was when I could not open an address in Navigon.