I no longer work at Microsoft, so please don't bother leaving a comment here or trying to contact me through my MSDN blog.
You can find my new blog at http://www.technologytoolbox.com/blog/jjameson. My new site also provides copies of all posts from my MSDN blog.
If, like me, you happen to encounter the following user experience when attempting to edit a master page...
...then follow these steps to resolve the issue:
Restart SharePoint Designer and you should be "good-to-go."