I wrote:
CFFI versions 0.9.1, 060514, and a build produced from a darcs pull today all show the problem on LW. I haven't tried bisecting any further.
Whoops, 0.9.1 works OK too. Don't know what I was smoking...
But this is somewhat puzzling, because the larger test case that I boiled this problem down to does have a similar problem on 0.9.1.