Brightboxes come pre-installed with Ruby 1.8.4, from the official Ubuntu Dapper distribution and have been extensively tested with many Rails applications.
There are a few known bugs with the 1.8.4 version, one in particular concerning parsing large yaml files might cause problems with your app if it does this, though this is not common. If this bug is causing you problems, you could try the 1.8.6 packages, though they are currently in beta.
We have backported Ruby 1.8.6 from Ubuntu Gutsy which is current available for beta testing. We do not recommend you use these packages in production just yet
You can find installation instructions here.