If they video in Photobooth it may help you limit their video time. Then they can import it into iMovie for the theme entrance. We use Vimeo to produce ours and save it as a mobile quicktime. Another place to send it to is Voicethread. But anything longer than 10 minutes needs to broken up into multiple segments.
Here is a blog that I posted about this. http://maistec.posterous.com/writing-in-math-goes-to-the-movies
Let me know if I can help!
The best example of student produced newcasts that I have seen in my travels visiting schools around the country is at Rocky Heights Middle School in Highlands Ranch CO. These are produced by sixth, seventh, and eight graders; but I was blown away with the quality of their daily newscast. They go "live" each morning with about a 10 minute newscast that call Nighthawk News. I am inserting several web links; I bet they could provide some valuable help. Good luck; the kids should have a great time and learn alot!
Here’s a great link on how to create a school newscasts on a small budget from HP’s Teacher Experience Exchange - http://bit.ly/gKrLyV
Hope you find it helpful!