RpnCalc is the best RPN calculator on the Android Market.
It has an interface that users of the HP calculator line will be completely at home with, including these features:
Basic (large key) mode
Key click (haptic feedback)
16-level stack (configurable)
Front four stack elements displayed
RpnCalc has a sixteen-level stack to hold more data. The front four elements on the stack are visible at all times, making it much easier to keep track of where you are in your calculations.
"Calculator tape" records your calculations and can be shared via email, bluetooth, etc.
See http://www.efalk.org/RpnCalc/ for manual