My hunch as I go into what a former co-worker described as "hardware is expensive-land" is that RoR scales via a "throw money at the train" algorithm. I'd like to be able to scale out layer by layer. Blah blah blah the point is, I am a java developer and there are more well-known patterns for scaling out large java web apps then there are for ruby. At a small company we need proven solutions that a) work and b) aren't too hard to do. The possible trajectories and migration paths at this time I see are:
Grails means I can move to an all Java platform using a very RoR-like idiom that will make life easier for the developers. Groovy 1.6 looks to have the performance problems of Groovy in the bag. It uses Hibernate, the one technology I feel like I have some strong knowledge of, strong intermediate if I were to use skier analogies.
Definitive Guide to Grails, Second Edition