Several links in the discussion below have ceased to work, so I have disabled them. You can now find the papers referenced in this post at Professor Conrad’s Ancient Greek Voice page at Washington State University.
If you are interested in the topic of Voice and the problematic issue of deponency, you should read Carl Conrad’s “Active, Middle, and Passive: Understanding Ancient Greek Voice.”
It’s available as a PDF download from http://www.artsci.wustl.edu/~cwconrad.
For further discussion of the same issues, see his “New Observations on Voice in the Ancient Greek Verb.” This 21 page discussion provides wonderful detail and clear reasoning. He raises compelling questions about the semantic import of the morphological distinction between what have traditionally been called the aorist middle and passive forms.
You can find the paper at http://www.artsci.wustl.edu/~cwconrad.
It’s wonderful to have both his and Pennington’s views on the topic available online for free!