It does not matter how many mobile web users there are in North America, Asia, or Europe. This has no bearing on how you design your site or what kind of technologies you choose to adopt. All that matters are the users of your site and what kind of devices they use. There can be 100x more smartphones in Japan than in Spain, but if all of your users are in Spain, then you should not care about Symbian's market share in Japan.
For a lot of sites, it is very easy to figure out where their users are because they are not localized. If your site is focussed on Brazil, then it's a good bet that most of your users are in Brazil. For sites that are international, things get trickier. This is a problem that I have had to deal with at eBay. Fortunately my job was easier because we already had a mobile site that was setup almost ten years ago. It is designed to support all devices, from old school WAP devices all the way up to iPhones and Droids. It has a presence in all of the countries where eBay has a presence. So there I had a lot of data to use where I could say "this % of our users are using iPhones, this % are using Blackberries" etc.
Let's get back to the @ppk's post and the iPhone for a moment. He makes a nice analogy to IE6 back in the day. When doing analysis on mobile eBay traffic and our strategy for the future, I actually made a similar analogy. There is one huge difference. The iPhone's browser, Mobile Safari, has been a front-runner in adopting HTML 5 standards. If you are optimizing for the iPhone, there is a good chance that you are in fact using HTML 5 standards. In other words, optimizing for the iPhone does not mean adopting a plethora of technologies that are proprietary to Mobile Safari. It's just the opposite in fact. This is the fundamental difference between Mobile Safari today and IE6 circa 2002. That is not to say that there aren't some technologies that are unique to Mobile Safari. There are. However, those are the exception, not the norm. Most of the code that you could write for Mobile Safari that would not work on other mobile browsers is actually standardized APIs that other browsers simply have not yet implemented. So yes, you could be excluding users -- this is where you need to figure out what devices your users are using -- but you are not putting up an iPhone-only wall. You are putting up a wall, but it is a "modern device that supports web standards" wall.