In a post to the Ruby-Lang mailing list yesterday, Yukihiro Matsumoto (more commonly known as 'Matz') announced the release of Ruby 1.8.5, the latest stable version of Ruby 1.8. Matz cites stability and reliability as the main benefits of the release over 1.8.4 and says there are no significant changes.
Mauricio Fernandez has put together a look at the changes between 1.8.4 and 1.8.5 and reports two new bugs. The most important is that Binding.of_caller is now broken and therefore the Ruby breakpointer (and that in Rails) no longer works correctly. Therefore, as with Rails releases, it's probably best you hold off on upgrading until the new release has been in the wild for a little while and workarounds to any problems have been made available.