Why Have PDAs Failed In The iPod Era?

Interesting disscussion on Slashdot – iPods vs PDAs.
In person I like PDAs more than iPods, but for now have only Sony Clie SJ 22 without MP3 playback. But if you want just listen music you need to count battery life on both types of devices. PDAs generally will eat battery more because of multitasking OS and more powerfull processors for example.