Google is reading your email
There are two things here that trouble me.
The first one is that I do sometimes forward a received mail without thoroughly looking through the attachments. Granted, I usually know the sender and the likelihood that some child porn found its way in there is actually very remote. But still. I expect my mail to be shared with the recipient and no one else...
Now, the first concern is a little theoretical. I'm quite certain that Google searching for child porn in gmail boxes is not really a threat to me. But once the box is opened, who knows where it'll stop? Will they be searching for terrorists? Political opponents? Will searching for "homemade bomb" in gmail be reported to the police? Where does it stops, really?
The answer is of course: we don't know. And that worries me.
For now, I'm sticking with gmail because I don't have time to switch to something else. But I'm already thinking about leaving, and I will at some point.