OK, here's an update on the situation. I had to abandon the EXSP24 AU idea because I just bought a new black Macbook but I found another solution at least for GarageBand. There is a way to play EXS instruments with GB. In a nutshell, Garageband has a built in sample player that it uses to play the built in GB instruments. All the GB instruments are actually .exs format (the same format that the EXS24 uses) so you just need to tweek the first line of the preset files and you can play any of your favourite .exs libraries without a 3rd party AU sampler instrument. The full tutorial is here:
http://www.xlr8yourmac.com/audio/garageban...ml#storytop%20)I've tested this and it works on my Hackintosh. I haven't tried it on my Macbook so YMMV. Just a hint for all of you folks that already have an extensive exs sample library; you will have to move your samples into the directory "/library/application support/garageband/instrument library/sampler/samplerfiles". A good way to avoid wasting disk space is to create an alias of the folder that your entire sample collection is in and put the alias in there instead.
I know most of you don't use GB as your main DAW, neither do I. I do however love to use it as a scratch pad for musical ideas because it doesn't need a dongle and it's super quick and easy to use. Anyways, I hope this is of use to some of you.