The cellular assistance just helps it get a fix quicker initially - it has no major bearing on on-going position updates once it has worked out its position, except again as an additional hint. If there's no cell info available, it can take longer to acquire the initial location/satellites, but it'll work fine from then on
My impression, from speaking to pilots and from my own ground usage, is that the Iphone GPS works spectacularly well (far ahead of any GPS I have ever used) when in receipt of a GSM signal, and only average when working alone.
The Iphone 4 is wonderful for running TomTom
