I recently stumbled upon a novel open source database engine which has been specifically developed to run in RAM: MonetDB (Query Processing at Light Speed). I believe this to be a milestone into what will become mainstream in as much as a decade : hard disk less computers . Hard disk less computers can be created even now by simply using a live disk of a distribution that offers it and instruct it to run in RAM upon installation . The use of some form of storage is still highly recommended , and for this pen drives can be used (currently going for about 4 euro/dollar per GB), so there is no structural reduction in the amount of points of failure . A pen drive lasts about 5 years, which is more than sufficient for most purposes and at the current rates should enable one to purchase 150 GB instead around that time for the same price .
I was never convinced about the argument of power loss either. It is already a rare occurrence, perhaps 3 times a year for half a second on average. Pen drives have a transfer rate of about 1GB per 5 minutes, so it would suffice to have about 15 minutes of supplementary power or even better: arrange to do delta-backups every few minutes, so supplementary power of 5 minutes would even suffice.
And there is also an environmental concern regarding keeping platters spinning as well as reducing weight of the machine as a whole.

I see this development as one more step in the direction of a high performance machine without a hard disk being the native configuration.