Imagine the following scenario, which I am proposing to a client for their service technicians' trucks.
I have a very small wireless LAN that exists only for the purpose of providing print services for iOS devices (using a Lantronix xPrintServer, a portable WiFi router, and a mobile printer, all wired into the truck's electrical system).
I have an iPad that is connected to the WiFi in the truck, and also has 3G service.
On that iPad, I go to check my email. What happens? Does it try to connect to the mail server over WiFi and fail? Or does it know it can't route that way and use the 3G instead?