Alright, so you went with the iPad 3G for that ubiquitous Internet connection? Well, it's there now, but there are some limitations. For example, while Netflix and Youtube will work over 3G (in low res however), ABC still requires a WiFi connection.
Why so? Well, it's very simple actually. On December 14th 2009, Apple informed developers that if they want to deliver video streaming over 3G, they must ensure their apps can adapt themselves to different network conditions. Netflix and Youtube did it (hence the low-res), ABC didn't.
Here are some bits from Apple's HTTP Live Streaming Overview:
iPhone and iPad apps that send large amounts of audio or video data over cellular networks are required to use HTTP Live Streaming. [...]
You must include a low quality stream of no more than 64 Kbps for your app to resort to when network conditions demand it, along with the higher quality streams you want to deliver to your customers when the network can support it.
This is also what AT&T pretty much told TechCrunch about the issue as well:
It's just a question for Apple
In short; the wifi-only deal is most likely just temporary. As for the low-res; it's only to avoid you too much buffering and not to kill the networks (1 hour of iPad Netflix over 3G = 250MB).
Bummed? Well, you can always try tethering your iPhone's data connection instead, work just fine and circumvents the limitations. Jailbreak apps like 3G unrestrictor will probably help as well once released.