He made the specific claim in the article that he believes the cost of switching operating systems escalates with time; if he sees it as inevitable that OSX will go walled, it's reasonable for him to cut his losses now.
I don't necessarily agree with him that OSX will inevitably go walled, but given that belief his behavior is sensible.
he believes the cost of switching operating systems escalates with time
Where's the evidence for that?
Desktop OS behavior has been converging for years. Linux gradually gets more and more features that were formerly restricted to proprietary systems (e.g. iPod compatibility). Emulators have gotten better. The hardware has converged: Everything runs on Intel now. The OS has converged: Everything runs with Unix underneath now. The browsers have converged: there's Webkit browsers for Mac, and Webkit browsers for Linux.
What makes it harder to move from Mac to Linux (or vice versa) than it was, say, three years ago, or five years ago?
What I really don't understand is how you can simultaneously argue that Apple isn't going to continue introducing new Mac OS features and that Mac OS is going to get increasingly hard to migrate away from. Stationary targets are easy to hit. It's moving targets that are difficult to track.
I think you're underestimating the effect of habit. As you work in an environment and become comfortable with it, you develop a workflow that supports that work. The longer you spend in a given environment, the more difficult it is when you finally do switch and have to relearn fundamental things.
Fortunately for those switching from Mac to Linux, several people have made the switch already (or use both simultaneously) and have written free software to mimic much of the functionality they came to expect with Mac and missed in Linux.
I won't pretend to know what the author was thinking, but one obvious answer is that the central authority approach makes data lock-in a whole lot easier.
Is there actually evidence for increasing data lock-in though? It seems to me that we’re all using compatible and often openly standardized formats for the files we create/consume on our local machines slightly more often than in the past, or else are storing data in web apps, where it’s accessible from any platform with a browser.
I think his worry is that Apple will extend their control over what software can be installed from the iphone/ipad/itouch realm to their PCs. That would dramatically increase the cost of switching.
The optimal time to switch would be after Apple announces OS X Evil Empire Edition, but before they ship it. Switching earlier (as Gillmor is doing) has no benefit.
I don't necessarily agree with him that OSX will inevitably go walled, but given that belief his behavior is sensible.