Memo Safe 2.57
the author of this app says:
This is a 100-percent compatible replacement for the built-in Memo Pad application. With easy-to-use memo encryption, security and tons of advanced features, this is the perfect add-on to your Palm Pilot.
Now you can read and write the same memo records as the built-in Memo application. It has every feature of the original Memo Pad along with the bonus of additional security. There is beaming support on Palm III or higher, find support, user configuration and a low price.
This also includes a viewer application for Windows that enables private viewing of encrypted notes.