The PDF was made directly from InDesign. The left image above is a screen dump from the PDF file, and the right image is a scan of the page of the book. When I intially reported the problem (to blurb.com) they told me they'd fixed it and organised a free reprint of the book. But the second book arrived with the same problem. I contacted them again, and they said they were looking into it. That was ten days ago and I haven't heard. The preview of the book on Blurb's website looks fine; it's just the printed copy that has a problem.
I suspect it's something in their RIP or how they impose. ID embeds fonts when you export, except for restricted fonts which this clearly is not or you'd be seeing a huge number of errors (and you can check that yourself in the PDF document properties). There is clearly a disconnect between the proof and the final output.