Unless you believe the Google fanboys, who think Google should exploit this to drive Android sales.
In the end it's better than the previous situation for iPhone users, because instead of a stagnant mapping application that wasn't updated for years for political reasons, we'll have competition now - as long as Apple is willing to accept Google Maps on their App Store, which I think they would, since there are many already (even a TomTom one).
The web based google map app is decent.
Google+ is a native app however, so I think they'll head that way eventually.
Sure, there's a cohort of iPhone users at the margin who may now choose Android over this issue (and we're hearing from some of them in this thread). But there's no leverage Google can apply with Maps that will significantly diminish the growing bulk of tens of millions of iPhone users. And Google surely wants that vast army of iPhone users to be engaged users of Google properties, including Maps and related features. Google Maps on iPhone is a net win for Google, and does nothing to diminish Android, where Maps will always be one step ahead and better integrated.
Of course Apple may have something to say about Google Maps for iOS 6...
You could even go as far as saying that mobile maps was the most important reason to create android.
Apple probably decided that having core phone functionality in the hands of a competitor wasn't such a good idea. Also Apple users were helping the competition with all the information they were uploading about geographic data.
When the original iphone shipped with google maps (and the Google CEO on hand), Apple and Google were partners, now not so much.