Revised advice for Mac users of ruby-processing

Update 7 December 2012

It appears that vanilla processing-2.1.0 may not be working on Mavericks, however processing-2.0.3 does work, especially using Apple jvm (but this is unsustainable in longer term, and I am sure you should be working towards using the latest Oracle jvm). It seems that you should really install the Oracle JDK rather than just the runtime (to get links updated etc) but if you have previously installed the Apple jvm make sure you do a thorough job of uninstalling it first.
I have just updated the main branch of ruby-processing (version 2.4.0) to make use of an installed version of vanilla processing. Until Ben Fry endorses processing-2.1+ on Mavericks, I recommend installing processing-2.0.3 on the Mac (and processing 2.1+ on linux and Microsoft windows).