Ayende: Yes I remember playing with this a while back and being very impressed..
Aspect# is one of the most stable last I looked, and built on Castle. It has some problems I'm not sure can be solved though. First, it screws with Reflection since it works with Proxies. Second, you have to mark everything as virtual if you want it to actually work, third, you have to instantiate through the Proxy factory.
Ok, the Reflection deal isn't so bad. You can write a helper to get around it easily enough. Second and third are real problems though, and the way c# was designed, and MS's position on bytecode manipulation, I wouldn't be surprised if we never see a truly transparent AOP solution for .net. :(