PDAXROM is just for the SL-C series at present. I don't know if they have plans to support the SL6000 but I'm pretty certain they won't bother with the SL5500.
Cacko distribution is a rework of the Sharp ROM with Qtopia Interface, not X based, uses Trolltechs QT Embedded framework. Most tranditional Zaurus applications use this framework.
OpenZaurus is traditionally OPIE based which is an open source remake of Qtopia, however, latterly you can run GPE on OpenZarus.
GPE is an X based distribution - the GNU Palmtop Environment - check out gpe.handhelds.org for screenshots etc. - I think this is your closest starting point if you want something really like a Linux desktop, however, you may find it lacking a bit in PIM functionality. - GPE includes Calendar and Addressbook stuff that with some fairly serious hacking can be made to synchronise using Multisync on a Linux desktop to something like Ximian Evolution. But you need to download quite a lot of the build framework to build this plugin.
Hope that helps as a starting point.
- Andy