I've been using a class called "MessageCapture" which lets you receive 
messages sent to an object. It's caused me no end of agony trying to get 
it working, so I thought I'd share my agony with the community :-P.

Source code here: http://www.squeaksource.com/DPON.html - click on 
"latest", MessageCapture-mvdg.2.mcz.

It's released under the.. umm... MIT license.

Also included is a Future class, which lets you do things like:

result := Future doing: [ some long computation ].

This will return immediately, forking off a process to do the 
computation and replacing "result" with the result when it's finished.

Some of the tests fail. Any hints / tips / guidance is very welcome! 
Especially as to how to capture #== and #class. Those two messages are 
sent by the interpreter even before doing a message lookup.

Michael.