I would advise against a PalmOS based device if you plan on doing development. The OS is outdated, and on its last legs. The iPhone OS, Windows Mobile, and the BlackBerry OS are all more modern and have better tools. Can't comment on Symbian.
If I were a developer of mobile phone software, I'd go iPhone all the way. Despite Apple's restrictions, every single iPhone owner would have easy and immediate access to my software. RIM's version of an "app store" looks pretty empty at the moment: