As far as I know, Apple already allows alternative browsers, but they all must be WebKit-based. This means that Google would not be able to use the Gecko rendering engine or IE it's mosaic-based engine. Add in the Apple's requirements for interface standardization, and any third-party browser would just end up looking like Safari anyway.
In fact, that's what a lot of web-enabled apps are, really: single-purpose web-browsers with a couple of custom buttons.