We’re only testing Formtastic with the latest Rails 2.x stable release. Patches are welcome to allow backwards compatibility with older versions of Rails, of course.
Development of a Rails 2 and 3 compatible version of Formtastic is underway in the rails3 branch, targeting a Formtastic 1.1 release shortly after Rails 3.0 ships.
Formtastic, much like Rails 2, is very ActiveRecord-centric. Many people are using Formtastic (especially the rails3 branch) successfully with other ActiveModel-like ORMs and classes (DataMapper, MongoMapper, Mongoid, Authlogic, Devise…) but we’re not guaranteeing anything at this stage. Patches are welcome, but it’s not our core focus right now. Shipping a solid 1.0 and Rails 3 compatible 1.1 is.