I am not a Rails expert, but if you are using Flex I would say that choosing Seam is a no-brainer. Exadel Flamingo tightly integrates Flex to Seam, and IMO Seam and Flamingo is a better solution than Adobe's own offering (Blaze DS). It is really easy to use Flex and Seam with Flamingo.
Seam beautifully handles RESTful calls. See the tutorial section 1.9: http://docs.jboss.com/seam/2.0.2.GA/reference/en-US/html/tutorial.html
Regarding messages... You can certainly use a database, but you should take a look at JBoss MQ (now JBoss Messaging) which is shipped with JBoss, and which Seam offers very nice integration.
Muito obrigado Felipe! This really solves almost all of my problems, and I'm really excited about Seam now. Regarding JBoss Messaging, I really need the chat transcripts (combined messages) in my main relational database to enable some advanced features. I'm not too familiar with JMS or JBoss Messaging; can JBoss Messaging forward the messages to one of the sender's conversation-scoped beans to be persisted?
And thanks again for your extremely prompt reply. Many of the other forums I've been on are not nearly as responsive or friendly.