Update: iPhone SDk 3.0 now addresses the question here, however the NDA prevents any in depth discussion. Log in to the iPhone Dev Center if you need more info.
Ok
The easiest way to get a Google map using Cocoa is to use the "Static Maps API". In practice, you need to prepare an NSURL that you use to contact Google. You get back your map as NSData, that you transform to an NSImage. Note that you can do both geocoding and reverse geocoding. You can also embed markers on the map. However, you loose the full controls you have access to if you use their JavaScript API.
Take a look at their reference guide here:
http://code.google.com/apis/maps/documentation/staticmaps/