I just updated my linux box to Fedora Core 4 and found that my C860 wouldn't connect anymore (over USB). After some googling around I found the reason why and the fix. I thought I'd post what I found here in case anyone else gets hung up on this as well...
FC4 ships with kernel 2.6.11. Apparently, at some point before 2.6.11, some "refactoring" was done on the usbnet module, which entailed moving non-standard stuff (such as the Z's non-standard USB net functionality) into external modules. In the process of doing this, support for the C860 (specifically) was broken.
This occurred because apparently the C860 is actually broken is TWO ways: it's USB net support is non-standard, like the other Z's, and it apparently also falsely advertises support for a cell phone modem interface called "CDC MDLM". The author of the usbnet module's changes wasn't aware of this and inadvertently broke the C860 support. A patch was submitted to fix this and that patch appears starting with kernel 2.6.12. The fedora repository has 2.6.12 posted and you can upgrade to it using yum: "sudo yum update kernel". I just installed it last night and my C860 is now happily connecting again.
If you want more details google with "MDLM C-860" or go here: http://lwn.net/Articles/130476/ and search for "usbnet fix for Zaurus C-860".