Stuart Eccles of liverail.net has posted a series of articles / tutorials showing how to get started with Flex 2 and Ruby on Rails.

You can view the articles here:

• RubyOnRails (1.1) and Flex (2.0): Pt 1
• RubyOnRails (1.1) and Flex (2.0): Pt 2

You can grab Flex 2 beta 3 from labs.