Actually, there are third-party services that have made a business of collecting locations of Wi-Fi routers and I believe Google subscribes to them.

They used to, but now do it themselves and then keep the database up to date with Android phones. This is why Skyhook is upset and suing:

http://gigaom.com/2010/09/15/skyhook-sues-google-in-a-locati...

well, the same is true for Apple who are now collecting location/wireless footprint themselves with iOS devices without Skyhook.

Yes, it's significant to note that Google (à la Street View scanning) is not the only party that's made a concerted effort to collect them. Whether Google's purchased (or absorbed) such data from others, I don't know.

I speculate that further routes for collection may be available. E.g. a user uses a wi-fi connection with a device that has geo-location turned on, and "Bob's your uncle".

I didn't know you could (legally) earn money for wardriving.

It's just 21st century map-making. Instead of plotting the location of physical landmarks, you'd be plotting electromagnetic landmarks.

I'm sure pioneer map-makers got in trouble for plotting perfectly visible stuff that was located on private property.