So the oft-heard caveat about not using SSD's for write-heavy applications (like the scratch disk) as opposed to read-heavy situations like OS/apps is a canard that may safely be disregarded? And you're saying you would go with SSD for the scratch drive before you would put one on OS/apps?
That oft-heard caveat is outdated FUD. OCZ Vertex writes well 250MB/sec while Raptors don't go over 200MB/sec, I think. Even if write speeds were similar, the practically instantaneous seek speed of SSD would favor them over HDDs.
As for scratch space, if one wants/needs ginormous scratch like Mr Schewe, it might make sense to go for HDDs. But for mortals with a limited budget an SSD gives best bang for buck.
Can't really say what would be the preference on OS vs scratch disk, as it depends on how much one needs scratch. With 12 gigs of memory I can't imagine anyone outside of e-peen measuring contests needing any scratch for non-stitched stills or non-video work. Of course if the OP is doing those then it's a whole different ballgame which I'm not familiar with at all.