Here's several for PCs or Bootcamped Macs: 
http://www.dofpro.com/
http://www.mootools.com/plugins/us/defocus/index.asp

I don't believe that there is a native Mac DOF post program. 

There is some real benefits to rendering in layers however. DOF can be dynamically updated and adjust without "destroying" the original art with a post filter tool.