Soliciting child molestation

I’ve seen many sting operations like “To Catch a Predator” and I have always wondered this legal question myself.

What if the adult posing as a minor winds up soliciting a minor? Apparently it happens. I think more often than is portrayed. The “bait” is always sure to mention that they are a minor, but the “prey” is usually not so forthcoming.

http://www.radaronline.com/exclusives/2007/08/xavier-von-erck-to-catch-a-predator.php

Legall speaking, when does this cross the line? I’m not asking if it is wrong or not.