I've heard of quite a few people still running Debian Sarge — the stable version of Debian before Etch went stable in April 2007. As per Debian policy, support for what is referred to as “old stable,” in this case Sarge, is slated to last for a year after the next Debian release is declared “stable” (Etch).
Read it here
But going three years without needing to do a reinstall is a pretty great thing. And if you start with a Debian release before it goes stable — like Debian Lenny, which is still in Testing but appears pretty darn reliable to me — you'll probably get more than three years. At this point, I imagine that most Debian users think of Etch — the current Stable — as too old. That's true for desktop users, but if your hardware likes Etch, I really see no reason to move to Lenny unless you want newer versions of all of the packages.