I have a 2009 Mini with Leopard installed. When I press F12 the first time, Dashboard opens. After I close Dashboard by pressing F12 and then press F12 again, Dashboard does not reopen. However, if I close Dashboard by clicking in an open area of the screen, instead of using F12, F12 will open it again.

On my Macbook with Tiger installed, I can open and close Dashboard every time with the F12 key. Also, Help on my Mini says that's the way it should work.