I use my iPhone a lot for browsing, nav, various apps, plus the usual email and things, and occasionally make phone calls with it. I don't read PM on it. The main reason for that is that it doesn't run Perl, and Apple won't let you make apps that run code. I mostly read PM when I'm in front of a regular computer with a real display and keyboard.

I will admit to reading technical docs on it reasonably often, and even learned how to make Wordpress widgets while sitting in a club in LA waiting for a band to come on. It's good for reading static docs like that, but PM tends to depend more on the ability to do some coding on the fly while reading it (which is possible, though inconvenient, using a terminal app). The iPad (just got one, as I got pulled into ebook publishing) is sort of more convenient for reading docs, but it suffers the same walled garden problem-- it's basically just a big iPhone that doesn't make phone calls and won't run arbitrary code.

If it exists though, I'll probably end up using it. It would be useful if it's flexible enough to display reasonably on an e-reader. I can imagine wanting to read something on PM and using a kindle for it, just to have the extra screen real estate.