Navizon offers some competition to Skyhook: This new service from Mexens Technology uses a form of peer-to-peer mapping. The seeding must come from people with either cell radios or Wi-Fi built in who also have a GPS (Global Positioning Satellite) receiver. These seeders can upload their information regularly, while all users of the service--currently free--can receive downloads of new locations when they synchronize.
The company also ties into Google Maps via its Web site to show registered users locations that have been mapped. Registration is also free. This is an interesting variation on wardriving which requires enlightened self-interest to succeed: Volunteer enough foothours of mapping and, if others do the same, a city could quickly be well-covered.
Navizon also adds applications, like finding buddies or (soon) tagging information to locations.
The missing piece in reading this Wi-Fi Planet article is whether once a city has a basic level of coverage that someone with just a cell radio or Wi-Fi could add additional points without carrying a GPS. Skyhook extensively pre-maps a location, but the company told me months ago that their software will report back new information to be integrated from software running in the field on users' computers, too.