Please contact support for this. There have been a few reports of similar and it would appear that something is reporting missing updates to OneCare, though a manual visit to WU is not detecting anything missing.
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If it fails to validate your subscription, select the option that you are using a trial or beta copy and you can proceed to email support without validation once you've signed in.
-steve