Rails 2.2 was released this week with some really interesting new features. Most interesting, to me at least, is "Thread safety and a connection pool". Rails applications are generally run using many single threaded instances through something like Mongrel. Now they'll be able to run multi-threaded, a huge leap forward.
What's most interesting, it almost only matters to JRuby users! For more information; have a look at Charles Oliver Nutter's post from a little while back: "Q/A: What Thread-safe Rails Means".