Well, the HD 5xxx series at the moment isn't even in the price range to threaten the HD 4890. When more mainstream cards become available then maybe.
Good point. With the 4800 priced the way they are now, ATi has the whole price range covered with very good cards available. The 4800 cards prices will not come down until aTi produces 5800's in their replacement, like a 5770 or 5830. Only then would you see a price drop in the 4800's