I would like to know the particular advantages of migrating to Jboss5 with regards to a grails application that uses GORM. I am running my application on 4.2.3 right now and am considering a more lightweight container since jBoss seems a little heavy for my needs. I really like jBoss and don't want to shift if I can streamline jBoss5.
JBoss can be as "light" as you want it to. I think the minimum configuration is like 80K and hardly uses any memory -- though it has no userful services running on it.
The first thing you should do with JBoss is figure out what you don't need and customize the server configuration to have only the services that you'll use. For example, if you don't need JMS, strip out the messing service. If you don't need web services, remove them. You get the point.
Peter Johnson and I cover this fairly well in JBoss in Action (which I'm plugging since we're doing a promotion here this week). We have a section in Chapter 15 on "Removing unwanted services".
JBoss AS 5.0.0.GA contains a new configuration called 'web'. This configuration is the bare-bones set of services needed to run a web application, and it also includes support for JPA. We cover this configuration in section B.1.4.