Now that the Mac/Ubuntu switch story has made it around the horn and back again (thanks for the non-link, Slashdot!), I want to clarify slightly what I meant by my assertion that Apple should be worried about "two lifelong Mac fans switching away from Macs to PCs running Ubuntu Linux" and that "nerds are a small demographic, but they can also be the canary in the coal mine with stuff like this".
Mark and Cory's switching is not going to send large numbers of Mac users scurrying for Ubuntu, no matter how well respected they are in a small community. Two is not a trend. But it may cause people to briefly consider that 1) the Apple experience isn't all that it could be, and 2) if you want a potentially similar experience, there's a non-Microsoft option available to you. And once that seed is planted, well, you know where that metaphor is going. (I'm also aware of a few other people who are pondering the same shift independently of Mark and Cory.)
In the late 90s/early 00s, Apple got their act in gear with OS X and their iMacs, Powerbooks, G5s, and iBooks. People who cared deeply about their computing experience (you know, computer nerds) took notice of Apple's rededication to producing great products, switched to Macs, and thereafter the Macintosh gradually became a genuinely credible option for programmers, web builders, graphic designers, journalists, students, and grandmothers. Not cause and effect, but the so-called alpha geeks noticed something happening and reacted before everyone else did. So when you have two people who care deeply about their computer experience and who were dedicated Apple users for non-superficial reasons switch entirely away from Apple for equally non-superficial reasons, it may be wise for Apple and the rest of us to take notice that they did so and, more importantly, why.