It is nice to have at hand a RPN calculator in Dasboard (Apple's Calculator at last offers this mode in Tiger, but dear what a wait!).
There is room for improvement :
-allow the configuration of keyboard shortcuts by the user (on my keyboard the key ~ does not exist and swapping the two top elements miss me a lot; may be the less useful DUP button could be replaced by a SWAP). This would brong support to internationalisation.
- a possibility to let users chose to display the buttons or not would be a plus (keeping the display area consumed at a minimum)