So does Ustream record the broadcast for later showings, and can comments be added every time it is viewed? I would love to use Ustream at school as a way to observe each other in the classroom, but obviously the rest of the staff isn't going to sit and watch the video at the same time.
Yes. You need to make your own channel and you can save your broadcasts. Backchannel is only open during the broadcast (I think), but there is a section for later comments under each recorded show.
One thing I hope they fix: the backchannel chats get deleted after the broadcast, so if you want to keep them, you need to cut-and-paste every so often.