Handheld platforms are changing quite rapidly now, and more platforms are becoming available as more devices become available. There was a time when selecting a PDA was a simple choice between Palm OS, and Pocket PC. However now there's much more choice both in terms of device and operating system.
In the past Palm was one of the few mobile OS's that had any kind of music applications available. Of course, some of the best apps are on Palm OS. Such as Bhajis Loops, Microbe, NotePad, BeatPad etc
Windows Mobile is perhaps one of the strongest mobile operating systems these days. However, when choosing a device be careful not to choose a Windows Mobile smartphone devices, as this will mean that you can't make use of many of the best music and audio applications. Of the many applications available for Windows Mobile there is of course MeTeoR, the 12 track multi-track audio editor and Griff, the music studio with numerous plug-ins. Of course the other thing to keep in mind is that if you want to run palm applications on a Windows Mobile device you can using the StyleTap emulator.
I know very little about Symbian, except to say that there is a tracker application available for it, and that now StyleTap are supporting a version of their emulator that will allow you to run Palm OS applications on a Symbian device.
Not so much of a PDA platform really, but it has not only a growing number of homebrew music applications, but also a number of excellent commerial applications, such as Electroplankton and Jam Sessions.
iPod Touch / iPhone
The jury is out on the iPhone platform at the moment, we'll have to see what the SDK brings in 2008 and how developers respond to it.
Palm OS II
Again, this new platform from Palm promises much, but is as yet to surface. If you believe Palm then you'll be able to use their new devices running Palm's OS II to run current Palm applications and also do all manner of new things. Who knows what it will mean for developers.
Nokia Internet Tablet
The Nokia tablet runs a linux based OS and the nice people at Access (previously know as PalmSource) have made a Palm OS emulator for the device. Ok it is currently in beta, but it does have potential. However, if you're looking to choose a device for mobile music, don't head straight for the Nokia as it has no native applications for music.
Last of Google's forthcoming Android mobile OS. No one knows what it will mean as yet. However, one developer (Daniel from Griff) has commented that as the SDK is Java based it will be no good for mobile music applications. I guess we'll have to wait and see,