good analysis, i never was quite sure how much data for the A-GPS (assisted GPS) approach the iPhone uses; i assume that's what it's doing ... not much
additionally, i did a test by swapping SIM cards w/ one of my kid's phone where i've turned off data service completely to the SIM card w/ AT&T (so no data charges ever) ... and even w/o data service turned on, my old 3G phone gets a GPS lock, a little slower w/o 'assist', but still achieved a lock.
to round out the topic of data use (beyond Navigon app) ... another important thing to be aware of is that some Satnav Apps are 'onboard' others are 'downloaded' during use ... so apps like, Navigon, TomTom, etc download all the map data when you install the program, and technically, don't need data service during use (other than assisting w/ GPS lock, or if you subscribe to live traffic service) ... and there are other Apps that download map data ongoing while use like Motion X or MapQuest and require more data during use.