Creative Creek made an excellent native HP emulator for Treo 650. Now they've released an HP RPN calculator emulator for the iPhone as a WebApp. But as a WebApp, I can't recommend it. There's an annoying split second delay between pushing a virtual key and when it registers. But at least there's a 15 day trial period before you buy. $10.
I'll buy it when the Apple Apps store releases a native version of this application.
Update Febuary 25th '09:
I've been using the native version of this RPN calculator for a few weeks. The size and function of the iPhone touch screen is perfect for this application. It works well. Now Version 1.2 has just been released, and for the meantime it's only $1!