Update: Putting plain vanilla straight quotes into the function does fix this. wptexturize properly curls the straight quotes for me, but if wptexturize were disabled (which I know some users prefer) the straight quotes would come through unmodified.
Oh, and sorry about how the code displays above. I obviously don’t know how to use the code tags properly, and when I go back to edit my post, the textarea is blank. Weird.