I heard from a friend of mine at Object Partners that JRuby was being used with Spring and Hibernate for rapid prototyping of applications, and today I stumbled across a blog entry by the guy that's doing it.
Bogle’s Blog » Ruby On Spring
Adam Waldal provides a short description of the work they're doing with JRuby. They've got it wired into Hibernate for database access and Spring for reusable services. The whole thing is fronted by Rubyfied JSPs, and it's truly a breath of fresh air compared to the typical top-heavy J2EE applications.
It's encouraging to see JRuby put to such good use!