Here’s an application note from Microchip: WiFly WebScan (PDF!)
WebScan is a special firmware release based on the standard WiFLY firmware that is designed to run on all Roving Networks Wi-Fi™ modules such as the RN-171 and RN-131. The Webscan firmware adds Real-Time Location System (RTLS) capability and is designed specifically to address the key requirements in asset tracking implementations, cost and simplicity.
The firmware is designed to operate in the tag that is directly connected to the assets, when tags wake from sleep, WebScan measures the RSSI levels from the local infrastructure‟s access points. The calculated RSSI values are then returned to the asset management server in a TCP packet. Using simple triangulation algorithms, third party software can then be estimated with reasonable accuracy. Servers may use this information to display the location information with shipment records or serve websites that offer enhanced mapping capability.