So, I tested the iPhones GPS. To make a long story short: doesn't work without cellular connection, is less sensitive than SiRF III.
To make it long again: I fumbled around with the GPS a bit. To compare results, I also used my Nokia E61i together with a Bluetooth SiRF III chipset based receiver and Google Maps.
Take a look at the picture. The accuracy seems quite acceptable, however, the inaccuracy on the iPhone is a bit higher (note that blue "circle of inaccuracy" that reaches beyond the yellow road while on the right it just touches it).
Beside of the accuracy, the sensitivity seems quite bad compared to the SiRF III. This is a bit surprising, compared to the specs of the Broadcomm chipset that is assumed to be inside the iPhone. I had to go on my balcony in order to get GPS positional info on the iPhone. As soon as I left the balcony, GPS positional info was lost. At that time, the SiRF III still receives positional info.
To make things worse, it seems that iPhones GPS won't work without a cellular connection. This might be an issue that is related to the Google Maps software, but still is a disturbing issue.
First of all, I put my iPhone to flight mode before using Google Maps. Then I started Google Maps and hit that crosshair button on the lower left. The spinner showed up, but nothing else happened.
After turning off flight mode, I almost immediately got positional info relating on cellphone towers and a moment later even a GPS position. So I turned on flight mode again. The GPS dot showed up in Maps, but it went grey.
As I didn't exactly know if the GPS worked in flight mode, I power clycled the phone, this time leaving the SIM locked. Again, the spinner spun but nothing else happened. Unlock the sim and GPS works.
There might be a third option, just having no cellular connection but still clear view of the skies. Unfortunately, I don't know where I could go to test that, because you get a cellular connection virtually "everywhere" ;)
But still, I'm strongly convinced that GPS will work with a cellular connection only. This leaves another question (and also a bad taste): What about roaming?
Update: I just put an other SIM module that isn't meant to work in the iPhone, so it wasn't able to log on to the cellular network (but, of course, still can scan its vincinity for cellular base stations and determine its parameters like BC, CI, LA and so on).
Still, I was able to use GPS! So, at least, you don't have to have an active cellular connection.
I'm still wondering where I could find a place without any cellular reception but clear view of the skies ;-))