Personally, I'm waiting to see what Adobe does with CS-5 Mac and then decide what to do about scratch. If they code it to actually utilize installed, available RAM, then scratch becomes almost unnecessary except perhaps for very large, layered files. And with some wishful thinking here, utilizing a fast OS array may be adequate. Worst case would be the same solution I am currently using -- the thin outer rim partition of a 4-drive stripe for scratch and the remaining for working image storage. Regardless, I may move my OS from its current 2-drive RAID-0 array to a single really fast SSD, or if they get cheap enough, a pair of smaller fast ones -- 64G x 2 is plenty -- in RAID-0.
If money were no object, I'd have OS on a 2x64G Intel X25E SSD R-0, scratch on a dedicated 4x32G Intel X25E SSD R-0, and image storage on a separate 4x256G Crucial SSD R-0 array. Of course that is like $5,000 in SSD drives, which is not out of the question for some, but still seems a bit extravagant for me. Maybe if I get a good bonus this year LOLOL!