I have two networks, a WiFi and a cable LAN.
I want use them together, but I want to force the iOS Simulator to use only WiFi, because, the LAN is under proxy, protected, with a lot of limitations.
The iOS Simulator does not get its own internet connection, it uses the computer's. Therefore, if you want it to use the Wi-Fi network instead of the LAN, just make sure you're connected through Wi-Fi.
In System Preferences / Network you can rearrange the order in which the computer gains Internet connection. Just place Wi-Fi on top of the list. On Lion it's done by clicking the gear thingy, and select Set Service Order from the pop-up list.