This is an obvious hint, but it saved me: If you accidently quit Word without saving, Word will automatically remove the corresponding AutoRecovery file. However, if your machine is being backed up with Time Machine, you can go to the ~/Documents » Microsoft User Data » Office 2008 AutoRecovery folder, and then use Time Machine to recover the AutoRecovery file.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090320030104819