This is how most NAND flash devices work already, as I understand it. I can't find the place where I read this recently, but I'm pretty sure that, say, an old 32 MB CF card is actually something like 36 MB for redundancy's sake.
Hmmm...that represents a 12.5% "buffer" capacity. I suppose that if it was to be viable for primary storage, 20-25% might have to be set aside for this purpose. IE, if the consumer wants a 160GB HD, the amount of Flash that would have to be in place would have to be around 192-200GB. Like I said before though, its greatest advantage might be as a bootup and OS holder. This represents about 10-20GB (depending on OS), which will allow for more energy efficiency, better bootup speeds, and less noise.