I think overall hardware obsolescence has slowed slightly, especially for Macintosh. Vista has accelerated aging for PCs a bit, but in my experience, OSX actually runs a bit better on older macs than earlier versions. I threw tiger on a G3 imac just because i had an extra family pack license, and it really sped up compared to 10.1.
You did all you can to delay obsolescence by investing in an intel based mac, the last G4 iBook won't last as well as your macbook will.