I'm sure it is. In fact, I've wondered if Google might try something like that (e.g. if/when they release an iOS app, expose some custom URL schemes for third-party developers to use if they want).

That being said, there is another problem. I can't see Apple looking kindly on developers deliberately routing around a system API to use a competitor's. Perhaps, given the current PR issues, they'd be forgiving in the app approval process for the moment, but I certainly wouldn't count on such forbearance.

I don't know... I'd think that as long as you're not using private api's its all good! :)