Thank you. As a Ruby on Rails programmer I couldn't agree more. Rails 3.0 will bring some maturity... programming to interfaces and APIs rather than implementations. I believe Javaland "got that" a long time ago.
When Rails came out, it reminded us that programming can be fun. Rails 3 will shows us that not only can it be fun, but it doesn't have to come at the cost of stable interfaces, reusable code, and the ability to not have to go back and refactor your entire project every 6 months.
But. (And that's a big but). I think you might overestimate the philosophical changes that are going on here. Seeing pipe dreams and unicorns where there are simply just incrementally improved ideas and work horses.
The rate of improvement in the Rails camp is not going to change because of anything fundamental to Rails 3. It might change because we're running out of big problems to deal with, but I also somewhat doubt that.
So that means that yes, you will have to change your application every 6 months, if you want to keep up with the latest and greatest. But you don't have to. You never had to. A Rails 2.3 application, for example, could have been chugging along just fine for the last 12 months (2.3.0 released March '09).