It took me a while to work out why I couldn't import my Kmail address-book to an Android (Samsung) phone, so here's the clue in case anyone else has the same problem.

PIMS breaks long "Notes" fields into 76-character records when exporting a .vcf file, with a space inserted as the first character of each continuation line. New lines in the Notes field are signalled by "\n" (without the quotes).

This can be edited out by opening the .vcf file with Kate and doing a global search-&-replace in regular-expression mode, where the search term is \n\x20 and the replacement term is null. I'd assumed regex searches wouldn't extend over consecutive lines, but they do.