This article in the Guardian UK, Dump your iPod, the mobile’s taking over suggests that mobile devices are going to supplant dedicated MP3 players as the pocket music player of choice.
I do think the idea of carrying a PDA, an MP3 player, a phone, and a text messaging device (crackberry) is unsustainable. Only the nerdiest of ubergeeks are willing to sling so many pocket devices from their belt clips.
The downside of convergence is that you can end up with a device that is optimized for no use case and is only adequate for all of them, but these problems are being solved. For example, there’s no way I would carry a PDA and a phone now. My HTC smartphone is great for both and even my dad uses a Palm Treo now.
It’s an interesting market to keep an eye on.