After a long and rather confusing discussion over at kitchen table math, I believe we've reached a consensus that Groucho's sentence, "I shot an elephant in my pajamas", is NOT an example of a misplaced modifier.
In the interest of helping Catherine's writing students, I proposed the following example of a truly misplaced modifier:
Some years ago I was listening to the radio and an ad for an amusement park came on. The slogan at the end was: "XYZ Amusement Park — it's just not for kids any more!"
Now there's a seriously misplaced modifier ("just"). They wanted to say that XYZ Amusement Park was fun for both adults and kids, but instead they implied that they don't even allow kids any more. Of course, they should have said "it's not just for kids any more."
Readers, if you have more examples of truly misplaced modifiers, please send 'em on in!