My primary computer --- holding up pretty well to the challenge of the iPad, mostly because the iPad makes it not quite so convenient to have stuff going in seven windows at once and attention paid to none of them --- is a PowerBook G4, which should tell you something since Apple stopped making PowerBooks around the end of 2005. Come March, it should turn five years old, barring catastrophic failure on some irreplaceable part. I deliberately chose the last of a discontinued line --- this was the last Apple laptop made before the Great Intel Switch --- mostly because I was about due for a new computer then and I figured the first generation of new Mac Intel laptops were likely to be the first generation of a new Mac product.
Anyway, I know that my laptop is showing signs of being ready for retirement. The big one is the hard drive being full up even though I can't point to anything specifically that I can or should throw away (and that with the awkward offloading of my iTunes files onto the Time Capsule as a remote wireless hard drive). In fact, a core dump of Matlab-clone Octave recently filled my hard drive, so I briefly had Zero K free space. I tried to take a screen shot of that, wondering what might happen, but before it could take a picture the laptop cleared out about 200 K of garbage. Too bad, I guess.
Still, I think it's now beginning to demand its retirement. Besides the quirky networking issues where it occasionally decides to not bother with the network (though not so bad as it behaves with 
bunny_hugger's home network), it's picked up some habits of extended pauses before swinging into action (possibly related to the hard drive's fullness). And the other day I told it to start Firefox, and it happily added Firefox to the Dock without actually, you know, running it. I tried rebooting and it froze up in the middle of that. I know a sullen worker when I see it; I'm one myself plenty of the time.
Trivia: After purchasing the Eckert-Mauchly Computer Corporation, Remington Rand had the EMCC division report to Leslie Groves, former general in charge of the Manhattan project. Source: Eniac, Scott McCartney.
Currently Reading: Three New Deals, Wolfgang Schivelbusch.