A critical vulnerability for Outlook has been reported, http://www.microsoft.com/technet/security/bulletin/ms10-087.mspx. It appears to affect All Outlook versions including Outlook 2011 for Mac and Entourage.
A specially crafted RTF email message when previewed or opened could allow an attacker to gain the same rights as the logged on user. If you have followed best practice and segregated administrative roles out the risk is somewhat reduced.
Microsoft have recommended to apply the update immediately.