You'll want to read it all, naturally, but here's a bit of her summing-up:
There is nothing unAmerican about the present perfect. We can and do use it in the ways that the British do. We just aren't restricted to it. There is something unBritish about using the preterit with certain temporal adverbs in particular and perhaps also more generally to refer to recent-and-still-relevant events. The difference between Did you eat yet? and Have you eaten already? is, in AmE, mostly a difference of formality, possibly also of emphasis.Imagine that. Once again, the language has failed (present perfect!) to go to hell in a handbasket. Thanks, Lynne!