If you follow Rails, then this weekend may have highlighted what some consider to be a drawback to the framework. Over the weekend Rails was updated to version 2.3.6 and then again to version 2.3.7. Most of these changes are in preparation for Rails 3.
All these version changes can become quite annoying, but luckily Wayne Seguin has developed the Ruby Version Manager (RVM) to help you keep your environments separated so you can have multiple versions of Rails setup, with their respective gems, and he has even made it extremely easy to switch between them using the RVM gemsets. So if you want to try out Rails 3 without killing your current apps, create a gemset for Rails3 switch to it, and you're all set. Need to go back to work on that Rails 2.3.5 project, switch gemsets and you're back to 2.3.5.
Here's the link http://rvm.beginrescueend.com/ As an added bonus, it is very well documented, so you should be able to get up and running very quickly.