I've no experience with Ruby. But, its fast productive results are interesting. For me as an architect it is interesting to know how Ruby development could be integrated into an pure JEE project that is based on a modern stack with JSF/Spring/Hibernate. We plan to integrate Flex the next year to skip JSF development the following years.
Yes. Ruby runs very well in an app server, as JRuby. JRuby allows you to mix Ruby and Java code, so you can have the best of both worlds. Also, you can then start developing new features in Ruby if you want. I've found that it's very good for developing web services -- REST can be very nice for that. You might look at Flexible Rails from Manning, given your move to Flex.
If you're determined to use Spring/Hibernate, you're better off using Groovy and Grails because Grails uses those two technologies under the covers, and in fact extends Hibernate by making a DSL around it in Groovy.
If you're using Rails or merb with JRuby, you'll discover that the Java EE frameworks aren't necessary.
I agree with Marc on the Spring/Hibernate points. However, I think a lot of people are hesitant to move away from Spring/Hibernate because of the time spent implementing them - which could be much less when using a framework like Rails. Rails for Java Developers has some direct comparisons of Struts to Rails, and Hibernate to ActiveRecord, should you want more info.