You've misunderstood Rails if you think it doesn't support building the UI first and extracting a model from that. That's exactly the way DHH likes to approach development. You can write a Rails app that doesn't use a database (e.g. Instiki, a great Wiki based on a pre-release Rails, that uses a RAM/disk backend).
That means you can cruft up a UI, store values in memory or whatever, iterate until you like your UI, and extract a proper model.
If that's what you want, I suggest you give Rails a try, Ian :) (Apologies if you already have.)