Google's My Location works better than an MPC/GMLC in terms of responsiveness and performance. I'm still not exactly sure how it works (Google won't tell me), but had a hunch it was a crowdsourced database of published information from millions of mobile users unknowingly volunteering to the system through Google Maps usage. Ted Morgan's comment on my theory, however, made me second guess these assumptions. With Skyhook already investing heavily in mapping wifi access points with drive-by-surveys, why not capture the cell side of the wireless airwaves? Consider this snippet from the Geolocation API description (yes, there's an API coming from Google...)

Many devices do not have native access to GPS or other location data. Additionally, GPS can take a long time to get an accurate location fix, drains battery, and does not work indoors. Because of these problems, the location API also has the ability to send various signals that the devices has access to (nearby cell sites, wifi nodes, etc) to a third-party location service provider, who can resolve the signals into a location estimate.

It's the third-party location service provider reference that's interesting – in addition to Ted’s comments…