From the Microsoft Office for Mac blog:
On April 12th, we released SP2 for Office for Mac 2011. The majority of our customers have been delighted with the improvements – new features and performance. Unfortunately, a small percentage are experiencing some issues with the update, specifically related to the Outlook for Mac database. We’ve documented the primary issue and work around in a blog post that can be found here.
Our goal is provide the simplest update experience for everyone – so we have temporarily stopped pushing out the SP2 update through Microsoft AutoUpdate while we investigate the issue. Customers are still able to obtain the SP2 update via the Microsoft Downloads site by clicking here. We encourage you to either wait for the AutoUpdate, or follow the directions in the above blog post before manually updating to ensure you don’t experience issues. We will provide an update once we have more information to share.