Rails 2.1 Released: Time zones, dirty, caching, gem dependencies, caching, etc
weblog.rubyonrails.org
weblog.rubyonrails.org
In other words, you mean "a point release," right? I know that many open source projects (I'm looking at you, Drupal!) may not care about this, but when I see a minor revision number change, I expect my applications and plugins to keep working without a major overhaul.
Hence, not a major overhaul. Perhaps a minor one. I usually take the first number to involve some large-scale shift in the architecture of the thing, the second number to be minor API changes, and the third to be bugfixes and security updates.