Turn your API users into data suppliers by adding an optional GPS lat/long and accuracy radius to the API request. You could potentially enhance the position they already have with wifi data (like Google Maps claims to do), and collect the GPS to wifi correlations to help other users. This is potentially a much larger data source than people running dedicated stumblers.
Yes, we are aware of this approach, thx for reminding us. It wasn't a priority so far, as we don't have anyone using the search API's just yet.