All it requires is a WIndows Mobile based handheld device wqith support of Wi-Fi and GPS.

The results can be saved in KML format which can be plotted on Google Earth/Map

AiroMap: Live in action

Features overview:
* Precise GPS Mapping algorithm (handles variable precision and loss of position data)
* Export to popular formats like OziExplorer, Google Earth and Garmin/Magellan/Navigon CSV
* Save/Load/Merge multiple sessions, using Airomap Log format
* Open/WEP/WPA detection
* Colors provide intuitive reading of encryption and strength
* No more hidden networks on map: slightly autoshift identical positions
* Audio signal if open networks are detected on your path
* Signal graph available for any network
* GPSID: no need to specifie COM port and you can use other GPSID applications simultaneously
* As small as 360KB. Operating System: Pocket PC / Windows Mobile.

Saving into other formats like CSV/Kismet have some issues but KML can do for most of the cases with some XML parsing.

airomap-screenshots

http://blogs.wefrag.com/divide/airosuite/