AlwaysOnGPS is a technology that successfully blends GPS positioning and Wi-Fi positioning together into one system:

When GPS signals are available, our software uses them to construct a map of the Wi-Fi of landscape around you. When GPS signals are lost or dropped (which occurs frequently in big cities, indoors, etc...) this WiFI map provides accurate Geo-positioning information for all your GPS based programs -- all the time and in every situation.

Try it on your pocketPC, there's a 30-day free trial here. An when one of the company engineers escaped for lunch: