As for a local / ftp synchronizer (as opposed to AB Sync's basic copy entire file as backup - instead of really SYNC), could I suggest that you use a VCF file format? Either one VCF file per contact, or a combined VCF. That would allow for nearly all phone's to be synced to and from TB, as most of them (especially iPhone & Andriod) can read & write VCF files. Even Nokia can work with these through the OVI suite. So effectively doing this would sort out much of the hassles many have with their phones & TB (i.e. having a "standard" answer of use GMail to sync your phone - WTF). The VCF files also work into TB through the MoreFunctionForAddressBook addon, and can be opened in most other contact managers (even Outlook works with them - go figure)!
Since the local filesystem sync is what I'm most interested in, I'll limit my reply to only that part of your comment...
I honestly don't care what 'format' is used, so VCF file(s) would be fine with me. I imagine it is fairly well established, standardized and stable? If so, that should make it even easier, since it would basically just be mapping the fields and tying in the 'sync' code (that compares everything to determine what needs to be synced).
