Nope. my iMac is a few years old and going strong. I don't even know where my iPod is. I haven't used it since I started listening to satellite radio. Even my Black Berry is several years old.
I do upgrade at most product cycles, but it depends.
I purchase computers online (Amazon) at a reduced rate. Update the RAM and HD, again at a reduced rate. Then I end up selling through craigslist to someone local. I usually end up at least getting what I paid for everything sometimes more. Those upgrades that cost me little, make it more valuable.
For iPhones, every since the first 2G 4GB model. I've sold for more than I paid for it. So basically I get the upgraded phone for free, plus have money left over.
Timing is the biggest thing. Sell as close to next product release as possible, not a sec later.