Is it possible to sync UbuntuOne contacts with Thunderbird's address book?
Michael is correct. Thunderbird doesn't integrate with CouchDB (yet :) so the only way to get it to sync with the Ubuntu One cloud is through our mobile phone contacts sync service since Funambol (our technology partner) has developed an add-on for that application. They also have add-ons for MS Outlook and the Mac Address Book applications.
Currently their add-on does not support Thunderbird 3 (so jumpnett is correct) but they're working on supporting that version of the application.
It is supported, but not quite as elegantly as Evolution syncing. Here's the step-by-step:
Basically, you must tell Ubuntu One you have a mobile phone (you can fake it) and then install the mobile phone syncing plugin for Thunderbird and point it at Ubuntu One.