I have had the same problem in Outlook 2007, and have it now in Outlook 2010 Beta:
It is however getting worse - every minute, in Outlook I get a pop up message saying:
"Cannot add to the server Junk Email Lists, you are over the size allowed on the server. The Junk Email Filter on the server will be disabled until your Junk Email Lists have been reduced to the size allowed on the server.
Would you like to manage your Junk Email Lists now?"
I have cleared all the junk mail lists, but this has not helped.
I use MS Exchange Server 2007, and I think the issue is connected with that,but I have no idea how to resolve it.