If anyone's interested in reading the applicable US Code...:
http://www.law.cornell.edu/uscode/text/18/922
Not ATF, but local police have done that.
A guy in (IIRC) Green Bay, WI was riding his bike through town, he thought he had carefully mapped where he was allowed to ride & was outside the magic unmarked 1000' line, the police disagreed, arrested him, charged him.
They measured from the school to where he was riding
three times, coming up with different distances each time, sometimes coming up over 1000', sometimes coming up less than 1000'.
The judge finally dismissed the charges, saying something to the effect of "if even the police, with measuring equipment, can't figure out where the line is, nobody else can be expected to".
Granted, all this happened before our cc law.