Thanks, Ronda, for the response. The referenced thread is certainly amusing, though I haven't read it all. I did get far enough to encounter the keyboard label adhesive theory, which I'd also found via a link offered by someone on another forum:
A Stinky Old iBook that Smells Like Sweat
http://lowendmac.com/menagh/06/1013.html
That seems to be it. This iBook has been in a drawer unused for a couple of years; apparently starting it up again caused a sudden flush of outgassing. It doesn't seem to smell so bad now. Well, nice to know what it is anyway. As it happens, I have a replacement keyboard for this model, which doesn't seem to smell (or very little); but it doesn't seem like I'll need it this time.