Two things I recently stumbled across that might be helpful:
1) PWx renders an alpha cahnnel - its been there at least since 2005. Any render to a TIFF against the plain background has an alpha channel. I did find that I had to go to layers>matting>defringe in photoshop to remove some scraps of the background color around edges of solid objects, but the translucent stuff looked great.2) It used to be hard to do this sort of rendering (look at truck):
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You need a floor for the shadow but then you have to light the floor or do something else to keep it pure white - and all that extra light would compromise your prodcut rendering. Since at least 2005 we can make our floor 'studio plastic' and change the color to white (to make it completely dissapear - translucent materials need to to be white to be truly clear). 'Studio plastic' will show a shadow but nothing else, so you can render against the plain white background to achieve the shadow in the sample image.
Of coruse the next thing to try is the shadow with the alpha channel - could be handy if it works.
-Ed