Hello,
I needed assertion to test my page caching logic (mainly the sweepers), so I created a plugin that add an assert_cache and assert_expire method to the test assertions.
The plugin page is http://blog.cosinux.org/pages/page-cache-test and you can find some information on how it works at http://blog.cosinux.org/articles/2006/10/05/testing-ruby-on-rails-page-caching.
Hope it helps somebody !
Regards,