It's not going to change, at least in the foreseeable future (and on this I can foresee a couple of years into the future). Allowing users to select arbitrary folders outside of their home for syncing with Ubuntu One, which could potentially sync between multiple different computers, opens a large number of usability problems to cover a use case that, quite frankly, isn't all that common.
One of the problems that I remember off the top of my head is that if you try to sync a mount point of a removable device (and quite a few people try to do this), when you remove the device syncdaemon will delete everything; to make it work properly syncdaemon would have to know about devices, detect their removal, things like that. Quite a large effort, and a lot of potential for usability nightmares.
Another problem is if you try to sync a folder with special permissions, ownership or file types in it: think of
/dev/ as some of the worst cases. Or any folder you don't own, really. We could simply disallow syncing folders you don't own, but we know for a fact some people are running syncdaemon as root (despite our warnings).
A workaround for you would be to mount (via
/etc/fstab, so you're reasonably sure the partition is mounted every time -- otherwise you risk losing your synced data) the
/data folder into your home. You could simply move
~/data or, if you have things that have the
/data path hardcoded (quite likely), or if you're already used to
/data yourself (also quite likely), symlink or bind mount
/data to the mount point in your home. If you don't want to see it in your home at all, just make it