When I first started getting into the hobbiest side of PCs, I'd open my case at least a few times a week -- in fact, it was something of a necessity because I could only get the thing to boot by clearing the CMOS on its budget Abit motherboard. Although I still count myself as a PC enthusiast, I hardly ever slide the top off my HAF XB and when I do, it's generally to clean or occasionally to troubleshoot, but rarely to upgrade.
There are many reasons why I don't tinker inside my computer as much these days, not least of which because it's simply less novel to me. I'm also pretty content with my setup as it is, so I don't really have a reason to start fixing things that aren't broken. It's the same for many of our staffers -- some of which don't even have an enthusiast-grade desktop anymore. How about you? How often do you open your case?
HAF XB image via Overclock.net