We frequently use comments in Word documents as part of our information security audit process and I finally looked for a keyboard shortcut to insert a comment.  The shortcut is Ctrl+Alt+M.  However, the most useful thing I found when looking for this was a comprehensive Word 2007 keyboard shortcut list at http://www.keyxl.com/aaa367b/5/Microsoft-Word-keyboard-shortcuts.htm.  KeyXL.com has keyboard shortcuts for all types of Microsoft, Adobe, Google, and other applications.  It's definitely worth adding a bookmark for if you're a fan of using shortcuts.