If you're building ActiveModel-like classes, it's quite useful that you run the ActiveModel Lint tests that's shipped with Rails 3. Unfortunately, if you're using Rspec 2.0, using the Test::Unit version doesn't fly as well.

I added a behavior in our rspec-rails-matchers gem which enables you to do the following:

Check it out at http://rubygems.org/gems/rspec-rails-matchers