APlogger is a native iPhone/iPod Touch application that scans and logs visible WiFi Access Points (APs).

It is similar, and owes much to, the popular iPhone Stumbler application, but provides a number of additional features:

summaries of the most recent scan and all past scans since invocation are displayed.
detected APs are logged in a tab-separated logfile for offline analysis. Logfile names are expanded using strftime().
the current location may be enumerated and is logged with each AP entry (sorry, no GPS support yet!).
one-off scans, or periodic “auto-scanning” at a chosen rate, may be requested.
the detailed view of the most recent scan may be sorted by SSID or RSSI.
the WiFi interface may be turned on and off with a simple control.

http://www.csse.uwa.edu.au/~chris/iphone/APlogger/