There are many Palm applications that allow you to store data in an encrypted form. Tools like KeyRing, Strip, and Cryptopad allow you to store passwords, credit-card numbers, and other critical information relatively safely, protected by a master password.
There are limitations to this approach, of course. The CPU in a Palm is rather anemic by desktop standards, so extremely powerful encryption is not feasible. However, unless the NSA is after your data, you should have plenty of time to change anything critical if your Palm falls into the wrong hands.