Posted here for my own reference for the next time it happens; occasionally, the icons on my Mac refuse to drag anywhere. Which is a bit weird. Binning the cache and preferences files listed in this post, then logging out and logging back in again, solves the issue.
Oh, and trashing QuickSilver‘s prefs fixes the problem where it disappears as soon as you invoke it. Yes, I have filed a bug report about that.
Shock! Mac OS X isn’t completely hassle-free! Gasp!