Origin, I've found with ps cs2 that I need to not use the Save for Web feature. Just a regular save. However, I've found GIMP easier and it also creates smaller PNG files, so I usually just save it as a PSD when I'm done in photoshop, open it in GIMP and then save as PNG. I'm sure there's a way to optimize better in photoshop and preserve the alpha layer, I just haven't found it.