All I know is back before I had the iPhone I didn't always take my iPod with me, but now that my phone is my iPod I never go anywhere without it. It's not really iPhone-specific; it applies to any phone that's a sufficiently good music player (if I remember correctly you have a G1, which would do nicely), and, as you point out, having Pandora on there (and I also occasionally use last.fm) makes it even better. My left front pocket is tight enough as it is holding my phone, my DS, and a pair of headphones; I don't think I'd have room to carry around a separate music player.