Mac OS X: execute a script or command when the network is connected

• ControlPlane (MacroPolo)
• ControlPlane allows you to build configuration profiles, contexts in ControlPlane lingo, for your Mac based on where you are or what you are doing.  ControlPlane determines where you are or what you are doing based on a number of available evidence sources and then automatically reconfigures your Mac based on your preferences.  Evidence sources can include your current location, visible WiFi networks, attached USB devices, running applications and more.  You can even write your own evidence sources using shell scripts!
• ControlPlane was created using code from version 2.5.1 of the MarcoPolo project still available at http://www.symonds.id.au/marcopolo/.  ControlPlane is a direct port of MarcoPolo and in fact, much of the configuration from MarcoPolo still works with ControlPlane, just better!
  ControlPlane supports 32 and 64bit Intel based Macs running Snow Leopard through Lion.
   
• LocationChanger
• LocationChanger is a simple shell script and launchd description. The script gets launched after every network change.
  When running it gathers some information, figures out the location and sets Location accordingly.