From memory on the torquebox (application server for jruby based on jboss) or the jruby mailing list there was one guy who worked for a large gaming studio (I think it was one of sony's gaming divisions) who use jruby and torquebox for the network backend[1] of their AAA games.

[1] Gamer / game analytics, logins etc. Basically APIs that their games call over the network for the more "social / analytics" stuff.

Personally I have used jruby in the past for implementations of services where we needed to use java libraries.