from what i heard, it seems one of the drawback for this particular carbon crank is the price itself. apparently it's among the most expensive crank ever. (btw, Competitive Cyclist once listed the price at approx USD 1.4k, but item has since been removed from Shimano's crankset lists)
some source stated Shimano decided to stop/delay this particular product in order to avoid a clash between the newly launch '09 D/Ace alloy crankset with the D/Ace carbon crankset.
nevertheless, the new 7900 D/A looks pretty and if i were to go back to Shimano, i seriously wouldn't mind the new D/Ace crankset