On the PlayBook, not only can you drag from the lower left corner to make the virtual keyboard appear, but inside an application you can still get the System menu to appear by dragging from either the left or right upper corners.
The top bezel "swipe down" action is reserved for applications to use for their own purposes, but they've left a region about 50 pixels wide at either end along the top edge of the viewable region where if you swipe down you'll get the System menu instead of the application's own menu.
So far, I can't see anything associated with the bottom right corner. [Update: although this may of course be different by the release date, or the third beta SDK, the bottom right swipe would trigger an orientation change. Since the simulator doesn't support this yet the swipe is ignored.]