Here's (http://rewrite.rickbradley.com/pages/moving_to_rails/) a nice article on the pros and cons of Ruby as compared to Java development. While it seems to be a bit too biased for my taste it still gives a nice overview and I agree with the points the author has collected.
On the other hand I asked myself if the guy who wrote this really wants to rewrite a large Java-based system in Ruby. If so, that kind of venture reminds me of the efforts to rewrite office suites in Java back in 98 (remember Corel?). It's like "hey, I've got this shiny new tool, now let's re-implement everything". Well, when you've got a hammer everything looks like a nail.
I remember Sun's campaign for "100% pure Java" and really don't think we need need a new dogma "100% pure Ruby". I go for "100% the right tool for the job".