How far will mobile music go?

I sometimes wonder if mobile music making software and hardware will be eventually consigned to the "interesting but nothing more" category. As laptops get smaller and more powerful and easier to move around with will there still be a need for devices that are even smaller.

The UMPC never really seemed to make it, but if Apple brings a sub-notebook to the market will that device, and others like it really take the lead in mobile music making? If, as widely rumoured, Apple do return to the PDA market, will that breathe some new life in?