Graeme has responded to my previous post. Let's start with where (I think) Graeme and I agree: The enduring difference between Rails and Grails is that Rails is built on Ruby, and that Grails is built on Groovy and Spring. Most of the other differences will be competed away, to the benefit of both frameworks.
So it comes down to this: In which layer should services and frameworks be built?
This only matters if you are primarily driven by architecture, i.e. you are Jane. Joe's worries are entirely different. I expect Grails to have significant growth and success helping Joe.