FYI: The BOINC-SETI-Astropulse beta test is accepting members again. Yes there is a Team MacNN setup.
I don't encourage participation unless you are one of those folks that likes beta testing and is willing to put up with the problems. Folding@Home is considering moving to BOINC eventually, so I figure it is worth dedicating 1 of my slow celerons to help improve the clients.
There is NO Mac Client currently, but supposedly they are working on one. Only Windows, Linux and (go figure) Sparc/Solaris.