The Wi-Fi positioning firm Skyhook Wireless gets a nice snapshot in the New York Times: I've been following and talking to Skyhook for many years, and the Times nailed the salient points quite well in a non-technical and useful way. The interesting figure is that Skyhook says it handles 250m requests per day for positioning data. There's a terrific video embedded in the article that shows a heatmap of location requests in Manhattan over a 24-hour period.
Most of the requests come from iPhones, Skyhook says. Apple has sold approaching 20m iPhones worldwide, and that means it's likely that the average iPhone users is pushing out several requests a day. I know I do! The iPod touch can also tap into Skyhook's system, but because the touch lacks a second channel to request information it must be connected to a Wi-Fi network in order to retrieve positioning data.