I heard that FCExtreme was going to basically be FCS with Shake included as a plugin and sell at some ridiculous price... along the lines of 10K+, IIRC. (I could be way off here).
Anyway, the huge ass price that some are predicting is what I find to be far fetched. Including Shake in FCS with a lower price, however, would be consisted with Apple's recent strategy regarding this software: combine and cut.
Really at this point its pretty much all rumor and speculation. The only things I can say with near-absolute certainty is that Shake is being phased out, and that something big is going to happen soon, with regards to Apple's pro video software.
In the meantime, the $499 pricetag on Shake is a good opportunity to get your hands on the cutting edge for a pittance of a price. Shake experience will certainly be useful in whatever's replacing it--Apple knows how much professionals love Shake, and as such I am fairly certain that Apple's next-gen compositing software won't be that different from Shake. ...so my feeling is, we should take advantage of the deal while it lasts.