does sound a cool idea, but i think theres a few flaws...
if you dont turn your pc on for a while wont the battery run out and you''ll lose data
ram chips are expensive for small capacity compared with hard disks!! you could easily pick up a 200GB sata hdd for the price of a 1GB ram stick! so maybe solid state needs to develop more before this is really introduced.. but it may start a drive for it!
it is neat though, simply the reliability factor!!