The YouTube app (free) for iOS was updated this morning with a new iMessage app that lets users share video and watch those videos inline without having to exit the Messages app.
When in the Messages app, users can tap the YouTube iMessage app to view a list of recently watched videos or tap the "Search YouTube" field to look for a specific video topic. Tapping on the video thumbnail places it right into the Message stream where it's possible to add a comment or just hit the Send button.
The Message recipient just needs to tap the app to start watching the video right in Messages. There's also an "Open App" link if the user would like to view the video in the YouTube app (see image below).