[Simon Brooke]

Hey, think this has been sorted. but to let you know (cs4 way of doing it) click the small arrow next to shape layer to open up all the options within… click on the arrow next to contents, click on arrow to open Rectangle 1, then click Rectangle Path 1, then within here you can drag the size settings to change the shape, position, and also Roundness to change the corners size. Hope this helps! Oh, I just saw you were asking about masks, yeah, this sucks. Stupid there is no straight forward way to edit the corners like shape layers (but of course you can use the shape layer as a mask and then regain all the edit options, but not a perfect way of getting round)