My ethernet connects to the LAN, my airport connects to a different network (it's another mac sharing a VPN connection). I want all apps except the iPhone Simulator to connect via the LAN.
Is this possible?
The main way you would have to go about this is probably using ipfw command line, although I really don't have much experience with it. You could make custom routing rules to do what you need to do.
Some other options:
You might want to try Little Snitch.
I'm not sure it has this feature since I haven't used it in a while, but it's worth the try.