This article in the Las Vegas Sun describes a Reno, Nevada man acquitted of chatroom child-luring because the 16-year-old woman he "lured" into an in-person meeting did not actually exist. She was actually a false persona created by the local sheriff's department. The judge ruled that you can't be guilty of "luring" a minor when no minor is actually involved, so no conviction; and the appeals court upheld that decision - continuing the US legal system's tradition of recognizing a difference between reality and fantasy. There is an added wrinkle that makes it less strong, however.
The thing is, the accused successfully argued that he didn't really believe the "victim" was under 18. She had apparently accessed his profile in the online chat system, and the privilege of being able to do that is reserved for persons over 18. (WTF? But design of virtual worlds is a totally different topic.) So, he claimed, it wasn't a case of "He thought he was luring a 16-year-old but wasn't really"; he argued that it was "He thought he was luring an older-than-18 person who pretended to be a 16-year-old". That gives the courts a reason not to convict him other than that the woman didn't exist in the first place.
If he hadn't had that defense, if he really had intended to lure a 16-year-old and had believed that that was what he was doing, would the courts have convicted him? There's no way to know unless they answered that hypothetical question specifically in the decisions. I haven't read the decisions but I doubt they would have answered it because courts hate to decide more than they're forced to. If the answer is "yes", then that sounds like punishment of thought crime, and a very bad thing.
UPDATE: There's been a second case in which the law was declared unconstitutional "for several reasons". Unfortunately not a lot of detail on those reasons is available, but here's an article in the Reno Gazette Journal. It sounds like the issue mentioned above for the earlier case, of the accused having a reason to believe the non-existent victim wasn't really underage, doesn't apply to the more recent case.