Your .png image needs to have larger dimensions, at minimum 1x2 instead of 1x1.
See: http://nemesisdesign.net/blog/coding/ie8-1x1px-semi-transparent-background-bug/
Internet Explorer 8 doesn’t perform
the repeat of a 1×1 pixel
semi-transparent background image
correctly when any other element on
the page is using the “-ms-filter”
drective for the alpha transparency.