I believe they are replacing the A900. In fact, they have already done it months back........with the A850. Ever since the release of the A850, all Sony was doing was running down the existing stock of the A900. Once the stock is used up, the model is as good as gone.
This is because, IMHO, Sony misjudged the importance of 5FPS (in a 24.6MP camera) which simply is unavailable in anything close to its price range and also the 100% Full-frame OVF, which too is unavailable in anything but all-out professional models. None of the professional reviewers or anybody else gave ANY credit whatsoever to Sony, for including these super-premium features that were only available in all-out professional models, at the price-point the A900 was being sold for. So all Sony is doing, is a "market segment adjustment", by retaining almost everything that came with the A900 in its "replacement" model, the A850, and downgrading a couple of its super-premium features (to the level offered by its true competitors - the Canon 5DII et al) by bringing down the FPS to 3 (roughly similar to the 3.9 in a 21MP camera like the 5DII) and bringing down the OVF accuracy to 98%.
As far as a purchaser is concerned, the A900 is a superb bargain and if they can pick one up for what it is being sold for currently, it is well worth it. And if they can live without 5FPS or the 100% OVF (which is the only discernible difference between the A900 and A850), its literal TWIN, the A850 should also be seriously considered.