In what seems like an honest mix-up, many review samples of ATI's newest video card, the HD 4830, are missing 80 shaders. This makes the card roughly 10% slower.
After some investigation performed by GPU wizard W1zzard, author of the GPU-Z program, it turns out that many review samples of the HD 4830 were specified to have 640 shader processors, but actually only have 560. Instead of having 8 banks of SIMDs, some review samples have 7 SIMDs.
Our HD 4830 reviewed here is missing the shaders, resulting in it not stacking up as well against the 9800GT.
It is unclear at this juncture whether this shader lack is only limited to the review samples set out. The video card only came out today in retail, so it might take a bit of time before this question can answered for sure -- however, reviews of the HD 4830 sold by board partners (Sapphire, PowerColor) have shown that they have the full amount of 640 shaders.
It seems like this a big accident on ATI's part. If the situation was reversed, and the review samples had extra shaders, that would be somewhat suspicious -- but there isn't any conceivable reason that they would want to sent out review samples that are slower than the retail cards.
Update: I've talked with ATI, and they seem very confident that the only cards lacking shaders were some of the review samples. However, if any retail cards are discovered to have only 560 shaders, this can be remedied with a BIOS update program that will become available.