So, google maps (for ppc windows mobile) has always had GPS integration, at least since the 1.0 release last year.
What they recently added is support for devices with integrated GPS, or even LBS GPS from the network. Currently only a couple phone offers this and are supported, but I am sure more will come, But the GPS function with an external Bluetooth GPS has always been there. If the iphone does a good Bluetooth integration it should be able to link with the com integration that is in the general google mapping function.