I have a shower curtain problem with a transparent image, too. This is a case where I had uploaded a non-transparent background jpg originally and then switched the files out for the transparent background png for the products. Worked fine for all but the shower curtain. There, even though I selected a white background and that's what shows on the main image page, when you click to open the shower curtain product itself, the image has a black background but a white border top and bottom.
http://fineartamerica.com/featured/migrant-worker-nikolyn-mcdonald.html
Mike was posting while I was and yes, that describes my problem, too. Abbie turned this over to the coders some time ago, but maybe they thought it was an isolated case? I told her it was not a huge problem for me as I'm perfectly willing to delete the entire image and upload again if necessary :)