I am currently playing with JBossAOP in JBoss AS 4.0. Cool thing!
You basically write a POJO with a method that follows a signature pattern, do some xml mumbos-jumbo and throw everything in the app-server, which deploys it. That's all.
There is one drawback though: currently it is not possible to intercept calls to methods on the EJB home interface. That's still the domain of the old JBoss interceptors that exist since JBoss 2.x.
But else, JBossAOP is really fun.