You guys seem to forget that you are not required to update to every release of OS X. 10.2 does not stop working when 10.3 comes out. You can install 10.1 right now, fire up Software Update, and get it patched up. Heck, I know people who are still using OS9. And as n0c said, updates are free, upgrades are not.
Personally, the only reason I would use Linux over Mac OS is if I did not have a Mac, and couldn't afford one, or couldn't afford software and had some moral objections to using pirated software. Other than that, most of the open source software will run in OS X with some tweaking, and you can use something like fink to get the functionality of apt. I think most of the software on Linux is incomplete junk (no flames please), and so in a practical sense your options are somewhat limited in what is available to you. Its just like Windows, in that there are millions of apps out there, but you'll only really consider the top 3-5 or so in every category, so that enormous app base is meaningless.
Now I've used Linux/BSD longer than I've used Macs, and the reasoning was cost, and the tinkering. These days cost isn't that big of an issue, but time is, so I prefer the polish. I spend most of my time in bash, but when I check mail or watch a trailer, I liked that it works without any fuss.