Stack Transparency Background only?

[Sannyasin Sivakatirswami]

Mac OSX; rev 2.8.1:

When setting  blend mode (transparency) of a stack, such that the 
background shows thru all objects are also transparent. Is there a way 
to make it so the background only is transparent? in other apps with 
transparency like this (I'm looking at the amazing quickSilver right 
now) the objects on the top are not transparent.