I have been very impressed with what Voicethread can do and can also see huge possibilities. Voicethread is a free website which describes itself as "A tool for having conversations around media". I came across the following Voicethread on the Japanese internment camps from WWII done by 4th grade students in the newsletter from Voicethread. This short video explains the project.
Here is the Voicethread:
Another one that impressed me was done by kindergarten students talking about trout hatchlings.
There is an educators' version of Voicethread which provides a more secure environment. There is also a comment moderation option so you can control who comments if you choose to invite public comment. You can find out more about EdVoicethread here and here. There is a free educator version which is worth joining as it gives you unlimited Voicethreads (up to a total of 2GB) as opposed to 3 in the standard version. I think it is worth upgrading to the Pro Educator option for only a $10 one-off verification fee. This gives you a lot more online storage space (10GB) as well as the ability to export archival Voicethreads.
There is also a Voicethread Ning and a Voicethread wiki where as well as classroom examples you can find classes wanting to collaborate on Voicethread projects.