In response to Eugene’s post below, about law enforcement possession and distribution of contraband, this post from my now-abandoned solo blog might be helpful. (Unfortunately the comments there are down, but you can get the gist of them — and particularly Marty Lederman’s answer — from the update.)
Here’s the key language from Nardone v. United States: according to Nardone, there is an “implied exclusion” for “public officers,” “where a reading which would include such officers would work obvious absurdity as, for example, the application of a speed law to a policeman pursuing a criminal or the driver of a fire engine responding to an alarm.” It’s not really clear to me how this would apply to the McDade case, and I don’t know if there are any later cases on how this applies (I haven’t found any, but I didn’t look very hard.)
UDPATE: Corey Rayburn Yung has additional thoughts at the Sex Crimes blog.