I change my icons to the "World of Aqua" set using CandyBar on my 12" PowerBook. It seems the "Watercolor" icons come back every now and then. I haven't noticed it happening only after a reboot or only after waking from sleep. There doesn't seem to be a pattern. Just every now and then, I'll open a Finder window and the blue Watercolor icons are back.
This doesn't happen on my iMac, although I'm running 10.2.6 on both machines. And I also don't have a clue as to why the 'Book would revert to the Watercolor icons and not the Apple Aqua ones. Watercolor was the last icon set I used, but why would that be saved somewhere?