In addition to Yehuda's comments, I would add that there are several major initiatives planned for the Rails 3 timeframe:
(1) Existing tests should be upgraded to be more thorough as the code is touched. There are various spots in the Rails 2.x codebase where tests could be improved, either because the state of testing has advanced, or because a patchwork of tests can be unified, or for other reasons.
(2) There's been some discussion of having a thorough integration testing script that can exercise the process of building and using a Rails application. This is at the "blue sky" level so far, but there's some work from Sam Ruby with automating the Depot application from AWDWR that points the way. Ultimately, this should give us a good working smoke test for Rails 3.
(3) The continuous integration servers that are used to test Rails itself are getting overhauled, with an eye towards getting us faster results on a wider variety of configurations.
So, the people working on Rails 3 are *definitely* interested in good testing, on a variety of levels. I firmly believe that as Rails 3 moves forward, we'll all be able to look back and say that it's the best-tested version of Rails ever.