It may be too much to start anuvver syntactical fred right after Ullrich's, but at this moment when I can hardly see over the piles of exam scripts lying on my desk, the question arises. Do you see a difference between the following two sentences?

A) "When I finish, I'll return the key."
B) "When I have finished, I'll return the key."

For me, these two are interchangeable, but some of my students claim that in A the person speaking has not yet started the activity, though they'll begin right away, whereas in B they have already started the activity. I think both sentences could be used in either of these two cases, with activity started or not.

What do you think?