I'd guess that the 200 f/2 L IS will sell for roughly the same or less than the 300 f/2.8 L IS. If that's the case, then it will drastically affect the market for the old 200mm f/1.8 L.
If you own a 200mm f/1.8 L, now's the time to consider selling while they still go for ~$4k. I'd bet that they'll drop a grand in price once the new IS lens is available.