The official TED app for iPhone and iPad has been updated to version 2.0. I've been watching a few videos with it and it's been a solid update so far.
The app is generally faster on 3G and videos load faster than the previous version. Living in a town where 3G is actually faster than my home DSL connection, I can confirm video buffering starts quickly. I'm a fan of TED's video player controls that sport the same metallic elements of Apple's Music app.
The big new feature in this update is the addition of subtitles and translations. Subtitles are available in over 90 languages and they can be enabled from the video player and they persist over AirPlay (useful if you're going to stream TED to an Apple TV or Mac running Reflector). Languages are available in a dedicated section in the All Talks tab; localized talks come with descriptions, title, and subtitles in your native language.
TED 2.0 is available on the App Store.