Wi-Fi positioning has nothing to do with GPS. It's a rather iffy technology that requires a database of wi-fi access points to be created and maintained, noting the location of each access point and thereby being able to triangulate your position. The location of each access point is verified manually "by deploying hundreds of data specialists who scan and locate access points using proprietary scanning vehicles designed to build and maintain the reference database" according to the Skyhook Wireless website. It can pinpoint your location down to a 20m/60ft accuracy which makes it relatively inaccurate, especially when you take into account the fact that APs are upgraded, moved etc. Keeping such a database accurate is a tedious task that requires a lot of work and a lot of funding. However, the accuracy may prove good enough for tourists etc.
It appears WPS only works in the US so far (and possibly London and Barcelona), Skyhook boasting a 70% US coverage (which I find highly unlikely, given the fact that APs are scarce in rural areas - I assume they may have 70% coverage in dense urban areas on a good day).
That said, WPS will be ingenious for positioning in major cities IF Skyhook manages to maintain and expand the database. It will NOT work outside urban areas, but on the other hand it needs no new hardware and will work on any wi-fi enabled device.
http://www.skyhookwireless.com/
EDIT: "WPS currently provides coverage to 70% of the American, Canadian, and Australian populations". So, most major cities.