Okay, this has been bugging me ever since they first visited Metropolis in Smallville. I just watched Thursday night's episode (yay for BitTorrent) and they sort of confirmed that their Metropolis is in Kansas.
I'll admit to not being an expert in the locations of DC's major fictional cities. I think that Central City (where the Flash hangs out) is supposed to be in the midwest, say, around Illinois, and that Gotham is, of course, New York-ish. Metropolis has always been, in my mind, anyway, in the northeast, too. Gotham's shining, opposite number, the bright, noble reflection of Gotham's sin and squalor, and sort of modeled after Toronto (if Toronto was in the United States).
So, just, you know...Metropolis, Kansas? Not sure if I can swallow that one.
(I know, I know. Fiction. TV show. Comic book. Shut up.)
The closest DC ever came to officially locating it on the map was a reference to it being "located near New York City and Gotham City on the East Coast of the United States."
Yes, I'm a big fuckin' geek. I know, all right?