I understand the concept, but I think that they've violated the user's expectation. On most sites, tabs do not support the back button, so why should they be different? The back button is what most users will click on when there is no other obvious way to undo their previous action, but in this case there is. As such, I think the expectation of the back button is to go to the previous site, not the previous video.

Hopefully, however, they did user studies and found that I'm wrong. I, obviously, have not done any user studies, so it's easy to believe that would be the case.

This is happening for me sometimes, but it is still sometimes broken. If I try to hit "back" to go from the 2nd video I view to the first video (on plain old reddit.tv), it doesn't work. Otherwise it does. (I'm on OS X FF3)

Back-button support without page reloads is one of the most notoriously difficult features to get working cross-browser....

the content they show seems to be pretty dynamic, so changing tabs is the actual action you're taking at first, going back will take you to your previous tag, and it will load your video then. Or with the anchor getting you to the videos directly, then the history works differently, since you're actually linking to a video directly.

these features are very subtle and useful, probably and definitely not the main focus of the site ;)