Some new and very promising features at VoiceThread: http://voicethread.com/about/blog/2011/20110216/
Already a good tool for easily-created presentations/conversations, the new M/5 commenting, allows you to move through a variety of uploaded content in one comment. Before, you had to comment separately on each new slide–okay, but limited. It looks like this new feature creates a smoother flow of your narration. I’ll have to think up something complex and try it out for you.