Tried re-installing both digikam and kipi-plugins, but I still have the same outcome - no kipi-plugins available.
The digikam splash screen provides a few progress messages among which is "loading kipi plugins", but nothing seems to result from this.
One clue might be this output which comes from running gwenview at the command line:
gwenview(14654)/kdecore (trader) KServiceTypeTrader::defaultOffers: KServiceTypeTrader: serviceType "KIPI/Plugin" not
This suggests that kipi-plugins should have registered itself as a KDE4 service (presumably during installation of kipi-plugins). There may be something in my KDE4 setup which is preventing this. I will try on a KDE list.