OS X is what made all the difference. Scientists love Unix, and the ability of this OS to run Unix and a high-quality graphic interface at the same time is what pushes them toward Mac.
Now if only hardware price/performance would catch up. A SETI work unit takes 7 hours on my G4 iMac, run from the Unix command ine (no graphics). On my infidel machine, an IBM R50 laptop running Windows XP, it takes 1 hour 55.