I ran into a small problem in PS3 with Smart Objects and masks - unlike regular layers, masks applied to smart objects are not linked. When you move the smart object, the mask stays in place. You can move the mask later, but imagine working with many smart objects, masking them, and later trying to move things around.
A quick Google search later I found a workaround
http://www.ianhoar.com/2008/02/22/photoshop-tip-1-smart-objects-and-layer-masks/
Here's a quick visual demo:
1. Gradient circle converted to smart object, made a copy, mask applied to look like Pacman
2. If you want to move the Pacman closer the mask stays in place.
3. Make a new group, put the Pacman layer in the group, select and apply the mask to the group, delete mask from smart object
4. Move the group, mask stays linked