There's a bit of back story. I run a Ruby course for intermediate developers at http://rubyreloaded.com/ and as a "bonus" part of the course I promised a run down of Ruby 1.9 in screencast format. My research showed there was more to it than I'd imagined so I decided to double down and put a lot more effort into it and sell it as a separate product too. So currently it's a video as that's what I promised my students, but I want to extend it later.

If it were a solely commercial decision, an e-book would have been the better way to go since there's still quite a bit of resistance against screencasts vs e-books IMHO.