Well, having finished unpacking everything at work a couple of weeks ago, and not found it, I knew it had to either be in one of the boxes or bags at home... or gone. So going through everything systematically was the key. It took a while, and it was in a bag on the last shelf in the last room it could possibly have been in, but I found it... in a white Apple Store bag that was otherwise full of CDs. Must've all come out of my car at some point.
It's the very first iPod model, holding 5 GB of music, i.e. about a thousand songs, and it's full of lots of stuff I didn't remember was on here! Nowhere near big enough for my digital music collection these days, though... I have about 8 GB on my desktop machine at work, and another few non-redundant gigs each on my home desktop and my laptop.
Seems to be working just fine... I charged it overnight and it's been playing for about an hour from the batteries, and the indicator still reads a full charge. I've got a likely buyer for this one, and my new one, a 60 GB colour model, will arrive in about a week. That ought to hold enough music. :-)