In order to explain the evolution of the third person accusative pronoun in IE languages, Mark Beadles makes use of the concepts of middle and medio-passive voices. But the discussion that followed his answer seems to show that these concepts are not so clear to everyone. So, what exactly is a middle voice (and a medio-passive voice, for that matter)?
You'll find a decent overview of valency-changing processes, with some comments on the term middle in Dixon & Aikhenvald (2000: secs.3--5, esp. pp.11--12), and references therein. Those authors suggest that the term is used with a wide variety of senses, and recommend that it be avoided (almost) altogether because of its ambiguity. Rather than insert a long block quote, I'd encourage interested people to just click through to the PDF and read the passage on pp.11--12.
In my favorite dictionary of linguistic terms, "A Dictionary of Grammatical Terms in Linguistics" by R. L. Trask from 1993, middle is given four meanings, the fourth of which is a synonym of mediopassive.
A mediopassive is a transitive verb with a passive meaning but intransitve form, syntactically. Examples from the book are 1) This fabric washes easily and 2) My new book is selling well.There aren't that many such in English but in other languages it is productive, like in Basque. Verbs that can be used both transitively and intransitively are also called labile verbs.