It completely borked, and screwed up my files. I ended up deleting all computers from the account, then cleaning up the carnage by hand.
When I wanted to reconnect the computers so that I could use it again (not going to trust it with anything I care about any more, but it is still handy for synchronizing bookmarks and notes), there appeared to be no way to add the computers back to the account.
It turns out that there's a set of command line tools
$ sudo apt-get install ubuntuone-
$ u1sync --authorize
will add the computer back in.