The Lilliputians have known all along that the Earth was flat. Of course they're only six inches tall so it's just more of Einsteins' Relativity Theory. Ever notice that if you look at a picture of Earth it IS flat?! (turn the picture and look at it edge on) It is curved on it's edges though.
Wasn't it filmed at Area 51? I think they have a sound stage and blue screen. (They have to use blue on account of the Martians.)
Geez, didnt you read the manual that came with your TinFoil(tm) hat? It explained it all! And it was filmed outside of Scotsdale. Not far from where the two shooters on the Grassy Knoll are buried.