In order to paste onto a layer mask (rather than creating a new layer) you must be viewing the mask itself. To do this, hold down Option (PC: Alt) and click on the layer mask thumbnail in the Layers palette. To return to the regular view of the layer with the mask applied, click on the layer thumbnail.
From Photoshop Finishing Touches
PS. You can also view the mask attached to Smart Filters in Photoshop CS3 Beta this way too!