Comment 4 for bug 463439

In the release notes:

A serious bug in the Ubuntu One client software included in Ubuntu 9.10 that could potentially result in loss of data has led to disabling file syncing access for this client version on the Ubuntu One servers as a precaution. Users who see a "Capabilities Mismatch" error when trying to use Ubuntu One should install the post-release upgrade of the client that will be made available immediately after release, fixing the original bug and restoring file syncing access to the Ubuntu One servers. Files are still available via the web interface at http://one.ubuntu.com./ [one.ubuntu.com]

Contact syncing and tomboy syncing services are not affected by this issue.

More info here:
http://voices.canonical.com/ubuntuone/