PDAs in have enough computing power, but unless the code is specifically written to handle the display and keyboard then it proabbly wont' work. Your choices are to find code written to run on specific hardware and acquire both the code and the device made for it, or learn to write code and modify some existing code yourself. Libraries of ballistic equations are available on the web but they won't be of use if you're not a skilled programmer.
Several companies sell PDAs and cell phones with ballistic software already loaded. If you have a portable notebook which runs XP or Linux there are some nice programs which are similar in performance to the JBM software which are inexpensive or free.
I like "GNU Exterior Ballistic Calculator" gebc by Derrek Yates I run it on a 10" Acer AspireOne which is about $300 and 2.3 pounds available at WalMart. I run it under Linux Fedora 11 (also free to download) but an XP version is available so it should run on most laptops or desktops. gbec is free to download. It handles G1 and G7 (and other) BCs. It has several nice features like the ability to store nearly unlimited setups for different rifle/cartridge/bullet combinations. It even has source code available if you want to make modifications.
All I can suggest for your IPAQ PDA is to Google if anything is available for it. No doubt you've done that already.