Some new iPad 3G users found out the hard way that streaming ABC shows on the iPad through ABC’s iPad app had some issues. You need to be on WiFi currently for it to work. ABC has already submitted an update to the AppStore and should be available soon.
In order to support streaming on the iPad 3G, App Developers must incorporate a lower quality stream in case you have poor 3G coverage. Since ABC rushed to get their app out I am guessing they did not put this in. They realize that people want to stream via 3G but in order to do so you will not get the high quality like WiFi offers.
So for now just sit patiently for the update and watch your shows while you are connected to WiFi.