It is just a local store. Yeah, maybe she was clueless. I thought about it after and I should of asked to try it on a D40 anyways, "just to see how the manual focus fee - oh is does autofocus, look at that?!"
Well, if you're still unsure if it will indeed autofocus, do a search for that particular lens and the D40 via google.
That should give you the knowledge you need.