I know that happens with commercial websites so I was wondering if commercial iPhone apps that use the MapKit (especially those that generate a lot of traffic) need to get any sort of license from Google Maps as well.
My best guess would be no, but just asking in case someone knows better. Also, I would guess Google gets license fees from mobile carriers, since some carriers charge extra for "maps and navigation".
Looking at Google's Terms of Services, it does not look like you have to pay Google for the use of MapKit with Commercial applications.
Google Maps Terms of Service (Specifically for the iPhone) http://code.google.com/apis/maps/iphone/terms.html
However, there are important restrictions such as you cannot charge a user for solely the use of a map (which most likely means if your app is just a locator application, you cannot charge for it).
According to the FAQ for Google Maps
http://code.google.com/apis/maps/iphone/terms.html
7.2 Service License. Google gives you a personal, worldwide, royalty-free, non-assignable and non-exclusive license to use the Service as provided by Google, in the manner permitted by the Terms.
7.3 Content License. Google gives you a personal, worldwide, royalty-free, non-assignable and non-exclusive license to access, use, publicly perform and publicly display the Content in your Maps API Implementation, as the Content is provided in the Service, and in the manner permitted by the Terms. Specifically, you understand the following:
The Maps API ToS here:
http://code.google.com/apis/maps/terms.html
will help. Basically they do say that they may decide to limit or throttle your requests to the API at any time at their discretion. You get a personal, worldwide, royalty-free, non-assignable and non-exclusive license.
It may be used for personal or commercial, with the exception that if it's used by a US Government department, there's a separate ToS for that too.
Essentially it's up to you to use for what you will, but their ToS gives them the legal right to kick you out or change the terms at any time. Although I've yet to hear of a case of that actually happening.
I suppose that the key part from TOS is 9.1:
...Your Maps API Implementation may not charge an incremental fee solely for the Service.
In other words, we can develop commercial iphone applications that use google maps if the app does more than what the google maps app does: a mashup. Right?