Answer: If it is one of many HP models, it is "regionalized". Ink cartridges you buy in one country won't necessarily work in a printer bought in another country.
That completely blows chunks.
I've wasted the better part of a day, not to mention five cartridges (I think), discovering that ink bought in the US will not work in my HP Photosmart D7460 printer bought in Italy. I should add that these weren't knock off cartridges or anything, but the real McCoy. This seems to be one of the least-known aspects of printer/ink/paper compatibility hell. It makes no sense, and unlike the idiocy of region protection on DVDs, there doesn't seem to be any information on the interwebs about how to reconfigure the printer.
Apparently there is a fix; call HP support here in Italy and persuade them to tell me the magic spell that will enable my printer to use their inks. But I have to be in front of the computer with the ink when I do so, which is going to be tricky until I have a day working at home.
I'm very peeved.