Sorry this hasn't been updated in so long. I am still working on this. If you're of the mind that aspects are trivially easy to do in Common Lisp, you're not wrong. I still don't have code here because I haven't made major modifications to my original hack in a number of months (read: a long time) and I'd prefer to polish it up a bit before I publish it.
I also took a different tack and tried to recreate the useful and interesting parts of Xerox PARC's RG effort. Accompanying the release of CLAS will be a paper (or a set of papers, I haven't yet decided) describing what aspects are, how I went about researching them, how I feel they affect Common Lisp, how Common Lisp affects aspects, what I ended up implementing and why, and finally, how you can use it.
This page is currently a synopsis of the Common Lisp Aspect System project, which I recently opened up to the Lisp development community, and indeed, the general software development community at large, with the introduction of a the CLAS project page at Advogato.
I started the project a few months back in 2000, and commenced research on a number of other aspect-oriented systems that have been implemented thus far, notably the work by the AOP group at Xerox PARC. Recently, my time has been gobbled up by other projects and I feel this is a constructive avenue to get a more diverse set of ideas infused into the project, with the hope of some high quality returns.
Although better explained elsehwere, the essence of aspect-oriented programming is that out of heavily object-oriented designs comes the problem of managing cross-cutting design issues: those concerns which necessitate a lot of small to large changes in a number of individual objects or classes. OOP methodology does not address this concern, AOP was invented and fleshed out to provide some assistance.
This page is still in a very embryonic stage, and will grow as the CLAS project community grows. For now, there's a list of resources I find useful. The code that I've written so far is undergoing some review before I turn it loose for public consumption, but that's slowed somewhat by my current travel and relocation schedules. A mailing list will probably be set up sometime soon -- people with resources to offer, please contact me at dnm@pobox.com.