Had the same problem on 11.10 as well. It has appeared once I've upgraded packages using Calibre PPA as Mark mentioned - but the source of the problem was python-dbus package (Kupfer shouldn't be affected by python-qt4 since it uses GTK). Once I've downgraded python-dbus from v1.0.0 to Oneiric's v0.84, Kupfer started to work.
Therefore I think you should take a look at python-dbus compatibility.
Had the same problem on 11.10 as well. It has appeared once I've upgraded packages using Calibre PPA as Mark mentioned - but the source of the problem was python-dbus package (Kupfer shouldn't be affected by python-qt4 since it uses GTK). Once I've downgraded python-dbus from v1.0.0 to Oneiric's v0.84, Kupfer started to work.
Therefore I think you should take a look at python-dbus compatibility.