Danne Lundqvist implemented a little library that introduces before, after, and around advice to methods. It's very simple: it just replaces the method slot of an object with a wrapper around the original method.
Noel noted the connection to AOP on the Untyping blog, and I've made some comments about the design of such an AOP facility on Lambda the Ultimate.
Update: Here's a prototype implementation of a variation on Ben's library. I implemented some of the suggestions I made on Lambda the Ultimate.