Originally Posted by
Bob'sYourUncle
Note of course that you can visit Mexico for much longer than that; US regulations are irrelevant there.
Correct, but what I think they mean is that you're supposed to turn in your form if you plan to be out of the US for longer than 30 days before returning. If you're returning within 30 days, you can re-enter the US on your original I-94 or I-94W.