There is plenty to draw from when it comes to using Active Voice (i.e. AV). It’s use is preferred over its evil step brother, Passive Voice, and therefore affords plenty of instruction on when and how to use it. Whether we like it or not, once in a while the Passive Voice (i.e. PV) enters our writing in such a way that it seems appropriate. Almost as if it belongs there. Sometimes it does! Now, before the rule writers roll over in their graves and begin clawing at coffin lids to escape and haunt me, let me explain:

In most cases, it would be better to write in the AV. Why? Because with AV, the subject rather than the object is the focus of the sentence. In fact, in many cases, PV allows you to omit the subject all together. And because AV is almost always more concise, vigorous, and authoritative which is generally the aim of all good writing: To say the most with the fewest number of words in the clearest possible way. There ARE times, however, when PV is not only acceptable, but preferred over AV–the Golden Child of the English language. Here are a few examples:

• When the focus is being done to something rather than by something.
  The wedding cake had to be carried by by eight strong waiters rather than Eight strong waiters had to carry the wedding cake. In this example the wedding cake, not the waiters, is the focus of this sentence.
• When the doer can be inferred or is not of interest. 
  It would be better to say The cake wasn’t served until two in the morning rather than The waiters didn’t serve the cake until two in the morning.
• To avoid using first-person singular pronouns.
  For example, instead of I randomly assigned the subjects to each group, saying Subjects were randomly assigned to each group. Here again, the focus of the writing is on what is being done, not on who is doing it.
• To avoid all-male pronouns.
  Instead of saying The average driver trades in HIS car every four years, you could say The average car is traded in every four years.
• To deliberately deflect responsibility or conceal information.
  Saying It has been alleged that Mr. Brandon knew about the takeover for months in advance is not the same as saying Mrs. Reisman has alleged that Mr. Brandon knew about the takeover for months in advance. That is, ambiguity in writing is not always the result of carelessness or inattention; sometimes it is quite deliberate.
• To vary sentence structure.
  Use it simply to avoid monotony. Wording every sentence the same way makes for tedious reading.

Paraphrased from Grammatically Correct by Anne Stilman ISBN 0-89879-776-4