In the past couple days the This Is Spam button has been inactive (grayed out) for new email under the Unread Mail folder in Outlook (2003). It is still present for the Inbox, but not any subfolders.
It is only present when looking directly at the Inbox. It is not present when looking at the new mail in the Inbox through the Unread Mail folder.
Other info: WinXP (32) all patches, NIS 2010, etc.