iPhone vs. iPod: Which is more useful? Which is more expensive, data-wise?

I think - and have thought for at least three years - that cell phones, music and video players, and portable computers (laptops, netbooks, what have you) will soon merge into the same device. Within ten years, and that's a generous range.

But in the meantime, I don't fully understand the various costs for data usage on web-capable phones and pocket pc's. Any feedback?