Internet connectivity on 3g iPad when on a non-internet-connected wifi 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?
When connected to both Wi-Fi and 3G, you have two IP addresses. One handed out by DHCP on your portable Wi-Fi network and one from AT&amp;T (or Verizon, as may be the case). 

To my knowledge and from my experience, your iPad will use the IP address obtained by Wi-Fi first, and then 3G. So if you're connected to a Wi-Fi network that doesn't have internet connectivity, the email address you're trying to check has nowhere to go since you don't have internet service connected to your portable Wi-Fi network. 

Unfortunately, this means that in order to check your email, you'll have to disconnect from the portable Wi-Fi network, allowing the 3G IP address to become active.