There are
some sites suggesting that the iPhone is not a phone at all, but a technology that will be introduced into new Macs -- ie. VoIP. I think that is more likely than an actual mobile phone for reasons MisterMe points out. It's a very messy market to get into and I don't know Apple is ready, or willing, to jump in.
Even if they were to get things rolling in the USA, what about other countries? Every county has their own mobile carriers and systems in place. It would be very difficult and expensive to penetrate the US market, let alone other countries'.
I agree with symphonix, too -- Steve Jobs hates imperfect solutions, and there are many imperfect things about mobile phones, many of which would be out of Apple's control.
I'll admit the iPhone rumors are based on more evidence than your average Mac rumor, but I'm still yet to see anything that convinces me.