... Why it caused problems in just one outlet on the other side of the house shall forever remain a mystery.
Just because it is physically at the other end of the house doesn't mean that *electrically* it is, also.
Based on your descriptions, I'm going to make the following assumptions (please correct any that are wrong):
1. The outlet where the wall wart was plugged in is on a separate circuit than the "lamp" outlet where you saw the noise.
2. There are no other outlets on that circuit (or you would have seen the noise at those outlets, too) - or you don't have any other X10 stuff on that circuit, so you didn't test there.
Are those two circuits on the same phase in the breaker box (usually, either the same horizontal row, *or* both are in either "even" or "odd" alternating horizontal rows)?
I've seen very odd noise issues affecting only a single circuit, but which was originating *outside* my home. Wire length on the circuit can be just right to make the noise create problems - and shortening the wire (which you would think would amplify the noise) might actually make the problem better in some cases.