I tried to use
Kindle for PC 1.0.1 and 1.50 with K4MobiDeDRM_v02.8_plugin.zip plugin for
Calibre on Ubuntu 11.04 x64 and it didn't work (for PRC files from K4PC 1.0.1 and for AZW files from K4PC 1.50).
In Windows it is important to use Python 2.6 x32. I wonder whether my 64-bit installation and/or other version of Python are to blame.