Looks like IE is drawing boxes wrong.
They came to the wrong conclusion on that other forum by the way.
What is happening, well, notice how it's only <span>'s that go onto the second line that have the problem, but not all of them. It's only the one's that go onto a second line AND the beginning of the <span> is not at the beginning of the line.
So while IE puts the bottom border and padding in the right place, it's putting the background image in the wrong top left corner. It's easier to communicate what I mean with an image, so take a look at the attachment.
As far as a solution goes, I'm not sure yet, perhaps something involving float:left;.