GPS and Network Positioning use 0 power if no app is requesting position information. If gps/positioning is active, there will be an icon in the status field, looks like a satellite antenna.

GPS depends on the receiver (N900) knowing the exact positions of the GPS satellites. The US military constantly measures the position of all the satellites, sends that position info to the satellites, and the satellites broadcast that information. However, the datastream from the satellites is very small, it takes half a minute or so to receive the entire position. Longer if the user moves his hand and blocks the antenna in the middle of those 30secs.

So, network positioning does two things, the position of the satellites is retrieved over the internet, which takes about half a second. Secondly, the cellular network can usually tell within a 20km radius where the phone is, if possible, the phone will get that position information too. This helps with finding the gps signals. Based on the current time, the orbital information of the satellites retrieved from internet, the software extrapolates where the satellites should be, approximately. Because of earth's gravity-well, and the massive speed of the satellites in orbit, their signals will be distorted and shift in frequency and time, to varying degree depending on their position relative to the receiver. With the N900's approximate position, the extrapolated position of the satellites based on N900's idea of the current time, it will be able to guess what the signals look like, and find them faster.

Once the signal has been acquired, it will use the time difference between signals from different satellites to calculate its own position, and you get a "gps fix" on your location.

So in short: Network positioning gives a rough inaccurate position in 1second, and speeds up gps high accuracy psoition acquisition by minutes.