I haven't really digested all the information in this article, but I'll post it here for future reference..
http://cacm.acm.org/magazines/2009/7/32091-the-five-minute-rule-20-years-later/fulltext
I'm excited for the day when I will have a flash tier to work with, and the perf increase it's going to bring, but articles like this make me think it's farther out than just slapping a bunch of flash together and plugging it in... 5 years?