Apple has confused a lot of people about what A-GPS is
A-GPS still works without a cell signal. You won't get as refined of a position as you would with the assist from the cell towers but it will locate you.
A-GPS requires a connection to get to an assistance server. So strictly speaking, without a cell signal the iPhone A-GPS reverts to regular GPS. Locating will be just as precise either way, since on the iPhone, the assistance is only the initial satellite orbits etc... after that, it's also just regular GPS.
A Touch with GPS could access assistance over WiFi, perhaps, instead.
Triangulation. You are able to get some sort of location based upon what towers you're connected to. That's how the original iPhone is able to determine a location but not a very good accurate location. Skyhook also helps as it uses nearby wireless networks to locate you.
Right, but cell and WiFi id are other locating methods. They are not part of A-GPS. When used together with GPS or A-GPS, the combination is called a hybrid locating system. AFAIK, each method is still independent in the iPhone, however.
Regarding A-GPS, The difference between an A-GPS and a regular GPS is that the former is Assisted by the cell network to find the satellites, making the initial locating much faster, but otherwise it's the same thing as a GPS, same accuracy and all
True for the iPhone, but A-GPS can mean different things. For example, for many years Verizon dumbphones have had A-GPS for E911 and VZNavigator, but most of them cannot act in totally standalone mode. Some even pass off all calculations to the network.
QFT. Please, there was a whole thread or five or twenty dedicated to the argument of how A-GPS works. It uses cellular or wifi triangulation to allow it to quickly find the satellites and lock on to your position much faster,
In the iPhone, A-GPS goes to an assistance server when necessary to quickly get initial satellite orbits, status, time, etc... and yes, perhaps an initial position guess. After that, it's basic GPS.
The folks behind Skyhook have come up with a combination method that does use cell id and WiFi to help get quicker GPS locks, but I don't think the iPhone uses it yet (otherwise they'd be advertising it). Especially since the cell id is done by Google at this time. IF Apple is using the new method, I'd love to know about it.