[Haskell-cafe] Why no IO transformer monad?

Hi all! For each basic monad there seems to be a corresponding transformer, e.g. 'StateT' for 'State' and so on. I guess the reason lies in the fact that if there were an IO Transformer monad, one might run it and get the 'Universe' out of the IO monad. But, I am working on understanding IO magic, on a much more concrete level, so I am am looking for a really good explanation of the above. (And the IO monad in general.) Thanks! /Henning