Lazerlike42
Darth Incompetant
I would say it's more likely that Palpatine knew about it because he also forsaw it and/or sensed it in Anakin. It is possible that Palpatine simply forsaw it/sensed it and used it his advantage, or he may have forsaw it and then planted the vision into his head.
I think either of those is more likely than his simply making it up to begin with because if you watch the vision in Anakin's dream, it is very accurate to the actual events. It is unlikely that Palpatine simply invented a vision and placed it into Anakin's mind only to have not only the general but also specific details of the event actually take place.
If I were to offer my opinion it would be that Palpatine sensed it in Anakin and possibly forsaw it himself. Remember that it is a vision of the future. It does not matter what events take place in between a vision and an event to arrive at that event; the vision transcends the details and skips right to the event. It is like looking at a DVD scene selection menu that shows short snippets of the scenes that are selectable. It doesn't matter what happens in scenes 1-10 to get to scene 11, if the little preview of scene 11 shows something, than that is what is going to happen when you watch the movie through.
What we are talking about here is somewhat of a paradox. It is in some way similar to this old time travel question:
If I were to build a time machine, and travel back to Mozart's time,
giving him the scores to all of his compositions, who wrote them in
the first place?
It's a paradox because it doesn't really have an answer that we as humans are able to figure out because the question transcends our existence in the 4th dimension. We are 4 dimensional beings, and thusly we have full dominion over the first 3 dimensions of length, width, and depth, and can understand and control every aspect of these dimensions. Our ability to think and exist in the 4th dimension, time, on the other hand is limited to a simple awareness that it exists. We notice it's passing, and we are aware that it passes, that is all. A 5 dimensional being, were one to exist, would be capable of answering such questions as the Mozart one and that of Anakin's dream.