>>But of course the law could change, or police could get more aggressive in their definition of child pornography.
Read the law for yourself, from the website of the Justice Department.
http://www.justice.gov/criminal/ceos/citizensguide/citizensguide_porn.html
It may be illegal even to have just one naked picture of a cherub on your computer, but the government doesn't seem to be cracking down on those people.
This may be similar to traffic laws. They beefed up prosecution of more dangerous crimes such as blowing through a red light and DUI, but they don't seem to be cracking down on speeding any more than they have in the past.
At the beginning of the coke wars they used to drag innocent people in for having trace amounts of cocaine on their money, until they found out that pretty much all money has trace amounts of cocaine.
Maybe its the case that every computer now has trace amounts of child pornography and cracking down on those who merely possess it would put just about everyone in jail.