Why have you taken so long to lay out your argument when you answer it exactly halfway through??
"The iPhone is only available in the United States (for now)."
Your statement is correct, and when is the scheduled launch of the iPhone outside the US???? Q4 2007 as I'm sure you are well aware.
Therefore the arguments you propose in your last two paragraphs are valid for, at most, the next 4 months. After which, the product will be released in upgraded form to countries with significantly more advanced, data capable networks in Europe and Asia. Potential target subscribers? Somewhat increased on the 233m in the US alone (ie. 3-4 times).
I have no idea whether the iPhone will then reach its target of 10m unit by the end of 2008 after that FULL YEAR of GLOBAL availability to a potential market of perhaps 1 billion (that would be 1% share, just to help you along).
It doesnt really seem quite such a tricky prospect as you imply though does it??