While SSD's are great, they're not the best choice for operations that do a lot of random writes and reads as in scratch. This is due to the SSD's performance degradation issues. Spinners in RAID-0 do not suffer this limitation.
Re the issue of scratch and data on the same spindles, I do agree with Farmer in theory -- however, in my experience with my machine I essentially never run into the issue of both operations tagging the spindle set concurrently. First, my machine set-up is an 8-core MacPro 3.2 with 24G RAM. I have an SSD for OS and apps, and then 4 SATA2 spinners in RAID-0 with a thin outer rim partition dedicated to scratch and the rest to image storage. On my machine, even when I'm doing a big pano, I typically process all the raws in C1, then read those tiffs into AutoPano. I have 24 gigs of ram in my machine, so those files all load, never tagging scratch. I wait for the pano to render, then bring the single large file into CS to perform my regular localized edits. About the only time I run into a double tag issue is when I'm batch processing a bunch of raws from a shoot in C1, then start working on a massive file in CS -- which is rare -- but even then I do not 'notice' the performance hit on my machine as the C1 batch is usually finished before I get very far along with my CS edits. CS5 will change this too, at least hopefully, if it utilizes onboard RAM more effectively. But as always, I respect that YMMV.
PS for Jeff: I would reconsider the SAS drive array. SATA3 is already here, and in a very short while you'll be able to load your box up with 4 @ 2TB SATA3 drives for probably less than the 4 smaller SSDs are going to cost, and it will likely be a lot faster. Just sayin...