Would you focus on Rails 3 directly ?
One remark: I would take care of sound quality and would not use the integrated microphone, but an external (even cheap, like head-set) one.
Feel free to contact me by mail (see profile) if you want to chat about your idea!
Right on. Crappy sound has made me turn off many a screencast (mostly informal ones, sure). The worst is when you can hear both the keyboard and a MacBook Pro fan whirring up because it can only just about deal with recording the screencast ;-)
A USB microphone like a Blue Snowball or one of the Samson things only costs $100 or so. Though even with a headset, if you know what you're doing you can run some EQ to make it sound bearable (Garageband is even good for this if you have nothing better).
Regarding sound quality, yeah, I'm using a headset.
Btw. I added you on GChat.