I've been reading SPCR for quite some time and it has always been the best information source I could find on HDD noise. Each day I'm using many HDDs in several PCs, some of them built to be the quietest I can get; obviously using many hints found on SPCR. At home I'm using Nexus Drive-a-Way enclosures for 2.5" and 3.5" drives (the best enclosure I found; SPCR review sadly missing), undervolted Nexus fans, modded Antec and Thermaltake cases, acoustic foam, fanless Yesico PSUs, fanless motherboards and GPU cards etc. By the way I sometimes spend a night taking sound recordings with pro/semi-pro hardware when I'm modding a PC. (..and, yes, girlfriend is asking me to enter therapy)
The review about the WD5000KS says "The 500 GB Caviar SE16 is the quietest 3.5" desktop drive that we know of on the market today".
I guess this doesn't take platter count nor gigabytes into consideration, only talks about 3.5" 7200rpm drives, and takes into account some average on measured idle/AAM/no-AAM noises, plus vibration rating.
Listening to the mp3 samples, I agree the WD5000KS seek noise is amazingly better than the 3.5" competition; however I prefer to focus on idle noise level alone, simply because it's what I'm hearing most of the time when using a 3.5" drive. I only hear worst seek noises when searching a file within a 20,000 files collection and a that time I'm ok (to some extent) to hear seek noise for a few seconds time.
I made AB comparison listenings of the WD5000KS versus the Samsung SpinPoint P120 SP2504C (cause I'm quite used to SP2504C and SP2514N noise signatures, having several of them; it's been my favorite 200-300 GB for the last two years).
Well, drive capacity not being taken into consideration, I clearly prefer the SP2504C idle noise. The WD5000KS has _much_ higher presence on mid-high frequencies, and this is where I'm most sensitive regarding HDD idle noise. I think this doesn't show enough in dB/A values. I made some freq graphs printscreens that you can see here http://idlenoise.ifrance.com/old
So, as some other people here I found the WD5000KS idle noise mostly average versus the competition. It would be interesting to have some SPCR review of a 3-platters Caviar SE16 (250 or 320 GB); maybe buying it anonymously in a store could help too.