Word-of-Mouth for Linux marketing?

Following-on from a discussion on the Linux Australia mailing list, Melissa Draper has commented on her blog on the effectiveness of WoM (word-of-mouth) as a marketing technique for Linux.  She raises Apple as a counterpoint to the “WoM is worthless” argument; Apple having spent (possibly) billions of traditional marketing dollars on Macintosh to achieve roughly the same desktop market share as Linux means that on a return-per-advertising-dollar-invested basis, Linux has a much better result.  I’m sure an interesting discussion will ensue; wander on over to Melissa’s blog and weigh in!