Egghead, I am bilingual and I also teach English as a second language. I think what you're talking about is kind of like the speaking/communication process. We are thinking about and choosing our words before we actually speak them. How long the time lag is depends on whether it's our native language or not. If it's our native language, we don't notice the time lag (although it surely is there, even if it's in nanoseconds!) If it's a second language, then the length of the time lag is determined by our ability.
Sight-reading is also the ability to look ahead in the music, granted different from playing an already-known piece.
On the one hand, I think there is something to pianoman's comment that hearing music in your head should be done away from the piano... Obviously this would apply if the music in your head prevented you from hearing your actual playing.
But on the other hand, I agree with what Egghead is getting at, we need to have an image of the music we want to create, and I think that image has to be as fluid as the real music, not some static still-life. That image, of the music we want to be making, should be going through our heads just seconds (nanoseconds?) before we play it, just the way we are choosing our words before we say them (however unconsciously or not)
Egghead, I'm sure your schizophrenic comment was tongue-in-cheek, but (sorry to drag the language example on) imagine what simultaneous interpreters do. They have listen to the speaker in language A, then after enough is said, they begin translating into language B, while *still* listening the speaker in language A for the next bit they have to translate. They are listening to one thing and saying another at the same time (because their translating is necessarily several sentences behind the speaker)
How about a conductor, who is listening to the music as a whole, listening to different sections/parts, and individual musicians, all at once!
Given all these examples, I think most would agree that focused listening does not necessarily mean listening to only one thing...