png transparency with PIL

[Robin Becker]

I'm trying to determine the transparency information for a png image. With gif 
images I can determine the colour that is used for transparent areas using 
im.info['transparency']. For the png images of interest there is no such entry 
in the info dict. I assume that's because of the way png does transparency. I'm 
guessing I need the alpha channel, but is there a way to get hold of it?
-- 
Robin Becker