Yup,
Hp is using Windows CE/Mobile whatever you wanna call it. Palm has
adopted Windows Mobile for the new devices as well. No choice, Palm
OS cmi. I don't think Palm is still deploying Palm OS, correct me
if I am wrong. Anyway, Windows Mobile provides better integration
with your desktop/laptop [if it is running the Windows
platform]
Symbian OS nowadays are popular among smartphone whereas Windows
Mobile are the main OS for PDA phones. Because I would say nearly
all stand alone PDAs now is penetrated by Microsoft. There are also
Symbian OS pda phone like the Nokia 9300 or those 9xxx series, and
the Sony Ericsson P800, P900 series. Symbian vs Windows, it very
much depends what you wanna do with it. If you want your phone to
be more pda like, choose Windows platform. If you want your phone
to be more phone like, Symbian is better designed for it.
I have Windows Mobile but not Symbian, may be the Symbian PDA folks
can share more.