I was able to get my hands on a Casio Cassiopeia E-200 for nearly no money at all. So the problem was using it with Linux. As I figured out, this is no problem.
The main program for using the Cassiopeia with Linux is synce. Synce is available as binary package in most actual distribution. Synce or for better understanding “sync CE” is the connection between any Mobile Windows(TM) such as Windows CE or Pocket PC. Synce does not do any installation or synchronization.
For the installation and synchronization you have multiple choices. There is kitchensync for KDE (synchronizes with Kontact, Korganizer, Kaddressbook) or multisync (syncs with Evolution, mobile phones, makes backups and more) for everything else.
A good source for (free) programs is www.freecabs.de or www.freewareppc.com.
This report is listed at TuxMobil - Linux compatibility guides for laptops, notebooks, PDAs, mobile phones and GPS. Also have a look at Repair4PDA.