The possible alpha channel of PNG format is not fully supported. I tried placing an alpha enabled PNG as the background image for <input> elements, and it just failed to render it properly. What it rendered was a white background instead of the transparency. The same bug appears in Gecko, too, but is fixed in Opera 9. There might be other elements that do not fully support alpha enabled PNGs.
Could you please provide a test case?
Ah I actually solved the problem I had to use background-color: transparent; I wonder why.