copy layer into a mask of another layer

Paste a layer(A) into a layer(B) mask. In 4 easy steps:
1. select the layer A and copy it to clipboard (Cmd/Ctrl+a, Cmd/Ctrl+c)
2. go to the layer B, make it active and click the Layer Mask
3. go to your Channels Panel, there is your mask in the bottom, set it active and visible, paste the clipboard content (Cmd/Ctrl+v)
4. make the mask channel invisible again and the RGB channel (top) active


From: http://pehaa.com/2009/02/photoshop-paste-a-layer-to-a-mask/