I was fiddling with migrations, and found these regarding the use of MySQL-ish ENUMs in Migrations:

• Enum fields in migrations? [rails.techno-weenie.net]
• HowtoUseSetAndEnumColumns [wiki.rubyonrails.com]

Basically, ENUMs aren't supported, and using Validations might be a good work-around, but is dependent on you putting (and keeping) all of the logic in your Rails app. If you need to go cross-app (with some other thing that ain't rails), then use CHECK or other DB constraints to enforce desired values.