OpenACS 4 will run on OS X. You can find an install guide here:
Unfortunately, new-file-storage is undergoing some
reconfiguration, so you may not be able to access this file at the
moment. You should be able to in the future. If you are in a hurry
you can email me and I'll point you to a private copy.
Note that there are a few additional steps that may be required
for 10.1. They are towards the end of this post:
Using the HOWTO as a rough guide, you might also be able to
get ACS 4 classic working on Mac OS X, but I haven't tried.
OpenACS 4 is currently pre-alpha, but some people are already
using it for production sites.