The problem isnt always insane data demands, but the morons speccing new systems with 20x146GB 15krpm SCSI drives instead of using lower-cost/higher capacity SATA drives.
20 x 146 is 2.9TB raw storage
8x 500 is 4TB raw storage...
a little googling shows the 146Gb drives using the same amount of power as a Samsung 500GB, with less than 1/3 of the storage... obviously the SCSI drive is much faster (lower latency times etc), but is that really needed for, say, a mail server with less than 10k users? nah.
An in-house built fileserver using high-end commodity hardware, 500GB SATA drives and Solaris/ZFS for storage will use drastically less power than a SCSI based setup with HWraid for a very small difference in real life performance. Obviously SCSI drives are rated for higher temps and longer MTBF, but with Raid5/6 (Raidz1/2 equiv.) its not much of a problem.
Speaking as someone who has 8+ years of experience with large-capacity datacenters and server farms, the problem with ATA drives (parallel or serial) in the past have been:
SAS drives in a 2.5" format are revolutionizing data storage, but the biggest problem is that in order to have a performant disk array, you need to have a lot of spindles, preferably in a RAID 0+1 or RAID 1+0 configuration. Raid 5 / 6 just doesn't have the I/O throughput to keep up with real-world load (especially in regards to web traffic and highly transitive SQL DBs). If anything, the biggest problem (at least for our servers) is that for SQL servers, there is too much disk space as the drive sizes have increased but we still need a high number of spindles for I/O performance purposes, so there are situations where we have highly transitive DBs that are relatively small (e.g. under 100GB) that are sitting on 1/2TB arrays. Unfortunately, all that space goes to waste. Until disk I/O performance improves to the point where you can get good DB performance from a relatively small number of spindles (e.g. 4-6), you'll be stuck with these large, wasteful datacenters.
On the other hand, some server companies (Rackable and Verari Systems, for example) are building plenum-style integrated rack solutions that are positively coupled to the (raised) floor and (suspended) ceiling that allows just the servers to be cooled and not the surrounding workspace. The servers themselves have fans, but also rely quite a bit on natural convection and 'chimney effect'. Unfortunately, HP, Dell and IBM have not gotten on the bandwagon yet.