"but am not quite clear on the concept
of "assisted GPS" (a-GPS)
There are at least four variations on what "a-GPS" means or how it has been set up, and basically it means "You dumb consumer, we don't want you to know squat about the man behind the curtain."
It can mean, and always used to mean "There's a half GPS receiver in your phone, all it does is receive signals, and you'll have to send them to the cellular carrier for computing and integration so you can't do anything without paying us an extra $10/month and being on a cellular data connection."
That was the first and is the largest meaning.
It can also mean assisted, as in "enhanced" GPS, where the phone has a full GPS set in it which is capable of being used independently, but the cellular network will assist it with more information, i.e. if you are in a parking garage or mall with no GPS signal, the cellular system will try to give you a position anyway. Or, to enhance the position (as WAAS does) outside.
It can mean your phone is or isn't locked to have the GPS function without an extra cost carrier plan. It can mean...whatever the carrier wants it to mean.
Last time I heard, AT&T meant it would enahnce the GPS function, and allow the GPS function to be used off-network as well. Of course that's subject to change at any time, and iPhones are only sold with data plans. For now.
Most cell phones also have little tell-tale stickers in them, when used in high humidity environments they will fail and the tell-tales tell the carrier "no warranty". Can't use 'em in rain or spray, you'll want a pouch or to keep it belowdecks. A real GPS won't have that problem.