pearPortVPN allows you to automatically establish a specific VPN connection whenever a wireless network of choice becomes available. Furthermore, it can also automatically disconnect any open connections before your Mac falls asleep and thus safe you from annoying error messages about timed out connections when your Mac wakes up again.
Now you can also specify an IP-Range (IP-Address with the * character as placeholder, for example 10.1.1.*) to automatically establish a VPN connection when connected to the Ethernet port and the obtained IP-Address matches the specified range.
TONYB212 This is an excellent little programme! Once the airport connects the VPN starts up automatically in 3 seconds flat - very useful automation. (Version 0.2.1)
ALVERYX Very simple but useful program that does what it's designed for. Easy to set up, sits in the Preferences pane out of the way. It connects as soon as you're on the network of choice.
Once thing that could be improved is that I wish it would reconnect automatically in case the signal drops out and then comes back into range, but otherwise it's good enough.