Okay, so I tested it. Here's a summary.
I started a new game. Gigantic map, frequent stars, rare planets, rare habitable planets. I set the scenario to Accellerated Start so as to get a leg up - you get 10,000 credits to start and a bunch of techs, including Warp Drive, which I needed for my strategy.
So first turn through, I created two classes of ships: The Sensor Bouy, which has 15 sensors and moves at 4. And the Interdictor, a small craft which has a single laser and two warp drives to move at 7. I bought a sensor bouy and set my spending to max out military, completely tossing research to the wind.
The second turn, I launched it and encountered a nearby minor civilization. I sold them all my techs, got lots more money, and proceeded to use the money to buy alternating Interdictors and Sensor Bouys, using them to set up a perimeter immediately around my empire, guarding my borders with the vigiliance of hawks. I also found a second star nearby with a habitable planet, and noted that none of the AIs were anywhere near my civilization. The sensor bouy set up station around the habitable star to the southeast, and I held off on colonizing anything.
Every scout ship that my sensors picked up, my interdictors flew out and destroyed. All of the AI scout ships had 2 sensors, and never got close enough to my home system or the second system to detect the second habitable planet. I made dearly sure of that, even though it meant ending up at war with every single civilization in the game. I encounter four other minor civilizations, each time selling all my techs to plunder their treasury, then destroying their scout ship they'd send to say 'hi'.
So now I'm sitting back, guarding my borders, making sure nothing gets through... and lo and behold, an Arcean colony ship pops up on my sensors, heading RIGHT FOR the single habitable planet in my area that NO scout EVER got close enough to see.
To sum up: It certainly appears like the AI is cheating.
Questions, comments? I have a save-game available if people are interested.
crickel
Edit: As a side note, the 'fog of war' being reversed is both: A. a plausible explaination for what is happening, and B. a plausible mistake. All it would take is accidentally leaving off a single 'not' statement in the code in some IF statement somewhere. If that were true, I would hardly call the programmers, 'the worst coders in the universe ever' or something like that. I've seen mistakes in code like that in place for years of use without being noticed.