First, I don't know if it's true. I'm reading it on blogs just like you. Aside from the occasional e-mail exchange, one phone call, being introduced twice (I'm sure he still couldn't pick me out in a crowd) and reading Robert's blog, I don't know him all that well. So I have no way to confirm. Last April 1st he tried to convince us he was leaving Microsoft. You can only play the April Fools joke once, so I take him at his word that he is leaving Microsoft.
Second, let me say that I agree that Robert has done a great deal for Microsoft. The biggest impact has been introducing blogging as a medium that promotes open communication. Honestly, I can't recall whether I became a regular reader of Robert's blog before or after I started blogging (since I'm not technical), but there's no doubt in my mind that as the poster-child for blogging about your employer, he has been an influence that has either directly or indirectly made many more people feel comfortable blogging here. In my opinion, that's goodness.
Now I have to admit that there have been times when I have disagreed with what Robert has said and sometimes how he said it, but that's kind of the point, isn't it? Free speech comes with a price; that being that you might hear something you don't agree with, something you don't want to hear. Same with blogging. Companies are made up of people, each with a different perspective and set of opinions.
People also have different dreams and passions. I think that Robert had the opportunity to fulfill some dreams here. Once that happens, your dreams are replaced by new dreams (otherwise, I'd still be obsessing over a pair of white boot roller skates with shiny orange polyurethane wheels). And I truly believe that Robert's move represents his quest to live some new dreams, ones involving his love of blog-related technology. Am I surprised that he is moving to a start-up? Not at all. He strikes me as someone who enjoys being an authority and likes new challenges. I also get the sense that, like me, he automatically casts himself in the role of underdog (another blog post coming on that soon). I see that fitting in with the start-up culture very well.
No company ever wants to lose good people. At the same time, people are people. There are many factors, professional and personal, that drive these kinds of decisions for an individual. I personally would never tell someone that they shouldn't pursue their passion. In interviews, when people describe their passion, I feel compelled to give them an honest reponse as to whether I think they will be happy in the role that they are interviewing for (the goals isn't hires, it's happy and productive employees). I'm pursuing my passion here (though I have mentioned before I feel strongly about free agency on the employment market...loyalty to a company must coincide with self-interest for the deal to work), though I would never, ever guarantee that I will be here forever, just that I'll be here as long as I am super passionate about the work I get to do and the people I get to work with.
So yes, there's a down-side to having one of your most public faces leave the company, obviously. At the same time, I say "Good for you, Robert....go get 'em!"
Oh I should probably also mention that this is not an official HR response to Robert's announcement (as if I'd ever give an official response), just my own personal thoughts.