In xubuntu 12.04 you can install
ubuntuone-control-panel-qt will then kick off the login and set up your synced folders and everything. Still no thunar integration, but the control panel has enough features that it makes using
For older versions, the previous answer below is probably still useful.
As @DoR said, you don't need
ubuntuone-client-gnome to use the file sync service of Ubuntu One on your desktop; you lose some integration, but you probably wouldn't appreciate that integration if you're on something other than GNOME anyway.
u1sdtool (part of ubuntuone-client ) is a commandline program that talks to
ubuntuone-syncdaemon over dbus, and allows you to do nearly everything you can do via the Nautilus plugin, and some things the Nautilus plugin doesn't know about yet. The only thing not implemented in
u1sdtool is sharing a folder with somebody via email; the sharing functionality exported via
u1sdtool is slightly arcane, and requires you to know the SSO username of the share receiver, and the share receiver to accept the share using
u1sdtool; I'd recommend you use the sharing exposed via the web interface unless you're very confortable with the commandline. The functionality currently in the tool and not in Nautilus includes the ability of unsubscribing from (and resubscribing to) some of your synced folders.
Nobody has done the work to talk to the same dbus interfaces from Thunar (either directly or via
libsyncdaemon), but it's certainly possible. Keep in mind that these interfaces sometimes change during development of an Ubuntu release, and while we try to notify other developers of upcoming changes, we make no promises. This communication issue has been the source of some significant frustration in the past, and we're working to address it, hopefully in time for 11.04.