A few extra data points
-Disclosure NetApp Employee, though not an official NetApp response, more of a personal rant -
@Anonymous - the $/GB of SAS increases relative to the $/GB of SATA, i.e. it's not going to fall at the same rate, e.g. SATA might drop around 20% vs SAS at about 5-10%. If the system was in $/GB you'd see a bunch of logarithmic curves with TLC flash dropping really fast and crossing over the curve for SAS but still being an order of magnitude more expensive than SATA. It might seem counterintuitive initially but this is actually one of the clearest ways of showing the relative costs.
@elReg - the "Enterprise" segment for HDD's that goes into arrays is about 1/10th of the overall HDD market, most of which goes into mobile devices today, though that too may end up being primarily flash at the rate things are going. It would be entirely possible for the existing NAND capacity to completely replace the enterprise disk segment even without new investment in fabs, which is likely to happen in any case even if it's just to satisfy the insatiable demand for low power mass storage in mobile devices.
@Jim - Enterprise devices have significantly lower failure rates in the field than run-of-the-mill OEM SAS/SATA, if you're on the hook for replacement under warrantee, then it makes economic sense, not to mention avoiding RAID reconstructs is a really good idea if you care about consistent performance in a shared environment.
@MyffyW & TwoEyedJack - Disk MTBF is a really bad predictor of Raid-set and array reliability (theres an IEEE paper I can point you to with lots of math and talk of Weibull and Poisson failure distributions and dependent vs in dependent failure data). Also the annual failure rate for enterprise class drives is significantly better than the stuff google was reporting on. Having said that, from what I've seen, eMLC drives are showing incredibly good reliability figures in the field.
SATA Disks will probably be replaced by something else for mass storage eventually, but probably not before any of us really care any more, but my personal bet is that it won't be NAND "as we know it" which is reaching the end of it's density journey over the next 5 years if the research is right, though you can make it denser, the speed will go down to almost disk like levels of IOPS / GB for writes. Having said that I know a bunch of smart people who disagree with me.