I have recently bought a Netwalker from Conics.net. The lack of built in wireless, limited memory & incompatibility of Qt/Opie with X being the main issues. So far this seems to be a very good upgrade from the Zaurus 3100.
I thought it may be useful to start a list of compatible hardware.
As a start: I am happy to report that the Ovation U727 works +/- plug & play. The only step is to do and eject + umount /dev/sr0 first.