The difference is definitely noticeable, but could be worked around - if you recreated the image without antialiasing, then you wouldn't get the hazing (which occurs because the image program smooths the difference between black and transparent with multiple levels of transparency - all but one of which become opaque in the single-transparency GIF). Then the difference wouldn't be very noticeable, and you'd get compliant code that looked good everywhere.

(When I need to create websites, I take a slightly pragmatic approach - since standards compliance doesn't ensure that it'll work everywhere, just make sure that it's intelligible, and that it should work pretty much everywhere.)

(I'll also refrain from posting my thoughts on those folks still using IE6...)