Photoshop :: Slicing And Exporting PNG With Transparent Background CS4?

A client gave me a PDF of a typical architectural floorplan; with various rooms in the building. My job is to slice it so that each room is a separate slice. Easy. But we need the background of each slice to be transparent. Essentially we want to overlay a line drawing outline of the rooms on a different background.But when I save it as either a GIF or a PNG, it always saves the bg as -white-, regardless of how I try to set the transparency colour.
 
Now in Corel, if you save as a GIF or PNG and set the transparency colour to the -background- colour (white), the GIF or PNG auto-magically gets saved with a transparent bg. But this is not happening for me.
 
So... is there a way to save my GIF, PNG slices so that the white is automatically converted to transparency in each slice?
 
Or do I have to -manually- remove all the white in the main image (not looking forward to that!) and then slice?