Someone at Mozilla Labs seemed to have re-written the Loki-Plugin from Skyhook Wireless to include the W3C Geolocation Specification - it is now both using that and the Loki-API to enable the browser to give location data, powered by the WiFi signal, to a webpage.
The whole thing is called Geode and will be included in future Firefox betas or can be installed into older Firefoxes directly from here. There is a blog entry and, of course, a TechCrunch article. Pownce and Fire Eagle have already integrated the specification into their services - and first tries show that it works amazingly accurate.