Frankyn is right, that's exactly what i mean
mr_killer> you're nearly done
-just select the zdepth(layer 1) then do a ctrl-A to select all
-do a ctrl-c to copy it
-select now the background image with the mask you've created
-go to the channels rollout, desactivate the RGB, Red,Green,Blue channels, select the last channel wich should be the mask you've created
as this :
-do a ctrl-v to paste the zdepth into it
-Reactivates all channels, comme back to the layers and add blur
-enjoy