I've been playing with Streamlined today, and, despite the never-restart-the-server impression you get from the movie, there are times when you get errors as you add belongs_to, has_many, and relationships to your model and streamlined files.
I pulled my hair out for far too long and was ready to chalk it up to buggy, prerelease code, when I just figured I'd try restarting WEBrick. Sure enough, that was all that was needed.
Streamlined is quite awesome, by the way. It's the missing piece that makes Rails absolutely fantastic. Now, it's a real turn-key application generation platform.