well, WebDAV IS iDisk.
All .Mac is is the iDisk & the mac.com email.
If you're looking for sync stuff, there are about a billion different ways you could do it.
For Address Book stuff, I'd assume you could create an LDAP server on Linux or something, and have Address Book look at that for the contacts.
Bookmarks for iSync, I really have no idea. perhaps make an AppleScript to load your current bookmarks file on the Linux server and everytime you add another bookmark, have it send a new copy to the server.
What exactly did you want to do?
enoch