HIPAA compliance is self-assessed by the organization (but subject to audit... just like income tax returns :-)
If you're actually storing PHI on your PDA as part of your organization's official records system, then your organization is responsible for telling what you have to do to appropriately safeguard it.
If on the other hand, you're using your PDA to access your organization's network via web or email, and PHI happens to be replicated on your PDA in your email folders or browser cache, I'd say that whatever policies your organization has for laptop computers would apply.
HIPAA (see
http://en.wikipedia.org/wiki/HIPPA#The_Security_Rule) tends to be very general. For example, wireless networks carrying PHI must be secured by encryption, but the exact method is unspecified. So, if your organization wants to use WEP encryption (which is relatively easy to break), they can, as long as they give reasons why WEP is adequate for their purposes. Somebody can steal your PDA and disassemble it to extract the embedded flash memory to bypass the device password and read the PHI, but maybe your organization doesn't worry about that (on the other hand, if you dealt with the PHI for the President, maybe they would).
I assume that blackberries and laptops are rampant in your hospital. What do your techo-dweeb doctor colleagues do?