You may have noticed the RPN support in version 1.6 - however the button layout really wasn't set up properly for Reverse Polish Notation.
Here's a little preview of the new RPN specific scientific button layout. You won't have to select this specifically - iSS Calc Pro will automatically display the layout when you select RPN calculator mode from the options.
The latest updates focus heavily on appearance and functionality. Not a lot of new features but you can now change many of the settings by simply tapping on the display area (ie DRG, base, output mode, etc). This version also improves performance and just generally improves usability.
I'll post a new beta soon and Windows Mobile Standard (smartphone) will be added in future versions.