Make sure you don't save over the psd from here on. That white background is on its own layer above everything else, correct? From there, you can merge all the layers except the white together (in CS2, select all the layers and ctrl+e; in CS I think you can link the layers and select 'merge linked' from the layers panel menu), ctrl+click the white bg layer, and delete from the merged layer. Then you can delete that white bg layer, and you should have a transparent bg. Use the crop tool to get rid of excess transparency, it should snap to the edges (if not view > snap), and then save as png with transparency.