I would like to implement a simple Bayesian classification system to do rudimentary sentiment analysis on short messages. Practical suggestions for implementing in Ruby would be
Ilya Grigorik has a nice answer to this problem over on this blog post on Bayesian Classifiers
Additionally, you may wish to take a look at the ai4r rubygem for some alternates to Bayesian Classifiers.
ID3 is a good choice because it gives a decision tree that is "understandable" to even someone without any real understanding of machine learning techniques.