http://appshopper.com/finance/17bii-financial-calculator
Current summary of HP calculator simulator/emulators available for iPhone:
HP-10bii, HP-11C, HP-12C, HP-12C Platinum, HP-15C, HP-16C, HP-17bii+, HP-21, HP-41C/CX, HP-42S, HP-48GX, HP-28C (partial simulation)
Many of the above are available in several versions from multiple software authors (including HP!). There are also many non-HP RPN calculators available.
Please add any to the list that I may have missed.