GPS and WiFi combination

Being interested in GPS topics, I've spent a few hours on Internet reading about the current available solutions. As I was browsing the pages, I've came across the Description of positioning systems. My curiosity has been directed towards Skyhook Wireless' Navizon - a software solution that allows mobile devices to access online database of positions of WiFi access points around urban areas of cities. For a mobile WiFi enabled device that is able to capture MAC of any access point, it becomes a matter of pointing down a known location of that device on a map stored locally. New maps can be uploaded to Skyhook's server updating the locations of moved APs. Likewise, maps can be easily downloaded to a portable device, allowing offline browsing.
In the absence of APs, GPS locator can take the lead. On the other hand, when GPS signal is low (places like buildings, tunnels), WiFi will kick in. The best part about it is that there is no need for Internet connection - MAC addresses are transmitted by APs for free!

In my understanding this is a great solution, providing the following:

• Maps can be downloaded whenever one has an Internet connection
• MAC broadcasting feature of APs can be exploited to determine one's position
• Can be combined with GPS receiver (use software to switch between WiFi and GPS)
• GPRS can be used whenever one cannot download maps via WiFi (i.e not available)
• Central database is updated by thousands of users automatically

One thing that is interesting to mention is the following. If a location of MAC address of some bogus device that is constantly moved around the city is uploaded to a server, it might create confusion because it can be detected by a mobile device in a different location and create the wrong sense of position, especially if it's the only or one of few APs around the area. Now, of course the software should be smart enough to detect the change and warn the user of inconsistency. All the user have to do is move and discover more APs.

Related:

RF Fingerprinting in Wi-Fi Location Tracking