Is something I use when I'm either very tired and must write long rather than short (paraphrased quote owed to someone like Mark Twain) or when I want to distance myself from the topic. Of course the passive is also used when it's the proper tense to use (heh).
But often the passive crops up as a disguise: "A woman was attacked" reads differently from "Someone attacked a woman." The difference is subtle (the former focuses on the woman, the latter on whoever attacked her) and may not matter, unless almost all crime reports are written that way. They often are.
Melissa points out a related phenomenon in her post about the recent murder of Yeardley Love:
"He then "shook Love, and her head repeatedly hit the wall," said an affidavit filed in the case."
...Because Huguely merely "shook Love," and "her head repeatedly hit the wall." It wasn't like he was shaking her specifically to knock her head into the wall! Geez! Her head hit the wall all on its own. I mean, maybe the police ought to be interviewing her head to see why it decided to hit the wall while Huguely was shaking her.