Aaron is correct, I am adding this answer to elaborate a bit and provide more details.
This is a huge thing in Rails land, and big (though not as bad) in ruby land.
The system version of ruby must be 1.8 or some installed applications will fail. There are quite a few UI applications that use ruby. However 1.9 is the new way to go, so you more then likely would want to use it on new projects (be they rails or pure ruby).
If your developing an application Ruby Version Manager should meet your needs. You can download and install it from this site. BUT if your trying to use 1.9 that was packaged for/with Ubuntu, RVM WILL NOT WORK
RVM only loads in your environment. Usually only in your command line environment (though this doesn't have to be true). It compiles ruby from source and does not use packaged versions. Again this is the accepted way to use ruby for development, not as a user or consumer, or in production.
If an app you want to run requires ruby, you should be able to give it a ruby path. If the app in question is built for Ubuntu then installing that app should be all you need.
If your working with rails
Do not use the system ruby or the packaged rails gems. They are totally and completely wrong for development. Instead use RVM + bundler + gemsets to manage your setup. On production boxes you would normally use just bundler.
If your working with ruby
Do not use the system ruby or packaged gems. rvm + gemsets are the accepted way to build new projects. In production this gets more complicated depending on targets.
If your deploying Ruby Apps
Then you need to make sure you run your test suite on the individual system versions that you are aware of. In ruby patch level is important and you need to make sure you code works, or is deployed with, a working ruby.
Instructions for compiling a working ruby from source can be found on my website. Note that it's for production servers, and intended to be used in rails environments. But it should get you started if you decide to compile.
- RVM + gemsets = developing ruby applications
- RVM + bundler + gemsets = developing rails applications
- bundler (and a complied specific version of ruby) = production rails applications
- system ruby + lots of testing = production ruby applications