The Office Engineering blog has a post up with an overview of the security features that went into Office 2010.  Here’s a snippet:

…we have designed what we have been referring to as a new security workflow, a layered defense that Office documents have to go through as part of the File Open process. We strive to make this process as invisible as possible. This means no noticeable delay in open times, as well as no dialogs asking you how you feel about security.

You can read the rest of the post here.