If you have successfully completed a backup and have not reinstalled or reset your database to resolve another problem, backup will remind you at every Tune-up by default. You can change this to another frequency, if desired in the backup plan settings - open OneCare, change settings, Backup tab.
If you reinstalled or reset the database, a new full backup will be required.
If neither of the above apply to you and OneCare acts like you've never done a backup, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve