UK broadcaster Channel 4 has released a 4oD Catch Up app [iTunes link] for the iPad, offering access to content shown on Channel 4, E4 and More4 from the past thirty days. Like the BBC iPlayer app, it streams content previously shown on broadcast television.
Unfortunately, as you'd expect there are some limitations. The app only works in the UK, and US shows are not currently available to watch; that's all down to licensing issues, naturally. It will only stream over WiFi, not 3G, and currently requires iOS 4.x, though Channel 4 does say an iOS 3.x compatible version is in the works.
There's also no archive content - i.e. older shows that you can access via 4oD in your browser, or those shows that were shown outside of the 30-day window - though, again, Channel 4 says it is "looking to bring these programmes to you soon." It's currently a free download "for a limited time only."
[Thanks to everyone who sent this in!]