This is a quick-and-dirty Mac OS X command-line application that registers with the SystemConfiguration framework's notification system to log changes to wireless state. For example:

% ./NetworkMonitor
Starting up NetworkMonitor...
[15:45:00]  Associated with <AP name> (<AP MAC address>) on channel 11.
[15:45:30]  Disconnected.
^C
Shutting down...

Tested on OS X 10.8.