The hard part about screencasts and interviews is keeping the viewers attention span. We've seen on Mixergy how the viewer count drops over time -- from Andrew's graphs he makes public. I'd be interested in checking out some Rails screencasts, but only if they're well edited.

How much of what you build is going to branch back out into front-end code? HTML/CSS/JS

Do you plan on making the code readily available to follow along? If so, how are you planning on doing this? Might be an idea to try using a dvcs. Assuming you plan it well enough, you could potentially have a bash script which will checkout your code at each video point for the user.