[akronate]

Have you tried MarcoPolo? It does context based switching of just about everything, and its free. http://www.symonds.id.au/marcopolo/

[Zev]

Yup. However, from MarcoPolo's page:

There are known issues with Snow Leopard:

WiFi evidence source and ToggleWiFi action do not work

Bluetooth may not work, or may cause crashes