Watching the panel discussion following Jordan Hubbard's talk this morning, I realized that the commonality between Mac and Unix users -- almost all hands raised when Tim O'Reilly asked about previous primary Mac desktop users and any Unix users -- was that Windows users toss their old machines, as old Windows hardware rarely handles upgrades smoothly for more than a version or two of Windows. You dump the system, resell it, give it away.
Meanwhile, Mac and Unix users wind up using machines til they die or get really really old. Most of the medium-power users I know wind up with networks of computers, running services or just as backups, given to kids or grandparents or neighbors. Adam Engst just mentioned on stage that he just turned his Mac SE off the other day, because it finally got to a point where he didn't need it.
I upgrade my Unix machines because the commodity PC hardware has actually died or gotten too slow for some of the intensive servers I use, but I've sold off or given away three of the old machines out of about seven servers. (Two are still running; two burned up, practically, and died.)
On the Mac side, a collection of friends and eBay users have all my old technology, much of which is still in use. My wife Lynn asked me how many Macs I owned the other day, and I said, "four, no five...wait, six. Okay, seven...eight if you count the Mac Classic II and the Quadra 700 that aren't running but are sitting in my office."