1src has a really interesting article on the PDA's supposed decline, and how manufacturers are starting a renewed interest in the format.
Add to that the recent comment by Apple that they see the iPod Touch as more than just an advanced media player and more like a new mobile platform, and the impending release of the SDK for this new platform, and you start to have some real momentum behind the old "PDA" format.
Add to that the fact that Microsoft wants to have Windows Mobile as a real consumer band, and the interest in mobile technology seems to be becoming a reality.
From a mobile music perspective I would have to say that the Apple platform is the most attractive, if they can get the SDK right and developers can make real applications for the platform.
It is an interesting start to 2008.