Photoshop troubles w/ transparent PNG

Ok, so I have some PNG files saved that have transparent background layers in Photoshop (CS2), but when I put them on a Web page, they refuse to remain transparent. I keep getting a white background. I've done this before without problems, but it's been awhile, so I might just be overlooking something simple...

I've tried saving as a PNG-8 and PNG-24 (File > Save for Web...) with the same results. The little 'transparency' box is checked... Thanks for any help