Ever since "hacking" my iPhone a few months ago, I've become an avid iPhone "gamer." The games are developed by individual developers for free distribution to limited numbers. Many are simple games like chess or solitaire. In other cases, they are outside-the-box designs like the iPhysics engine that allow drawn shapes to become objects with a virtual mass that move and interact based on the movement and position of the internal accelerometer. And that's going to be a key aspect -- in terms of capabilities for portable gaming, the iPhone is to the PSP or DS what the Wii was to the PlayStation.
When it comes to hardware distribution, the iPhone has only a fraction of the mobile phone market. But in terms of a gaming platform, if the company can hit 10 million units by the time the SDK hits in June, it will parallel, almost identically, the sales for the Wii over its first-year period. So for sheer numbers of units, the worldwide distribution of the iPhone isn't too shabby when considered as a gaming platform. And that's only for the iPhone, which is tied to a carrier contract. Since these applications will also be available on the iPod Touch, the total install base is not limited by carrier relationships.
The final "nail in the coffin" is the low barrier of entry for developers. With a totally free SDK, anyone and everyone can develop applications and games. And with the 2.0 software release, distribution will be done right on the phone, using an AppStore application (which is extremely similar to the highly successful application Installer currently used on hacked iPhones). And distribution through this application is completely free - though Apple will take a 30% cut if developers choose to charge for their apps. The only cost as far as Apple is concerned for developers is an initial $99 fee for a digital signature before being able to distribute developed applications. This cheap, centralized distribution should be of particular note for companies interested in the development of advergames.
Ultimately, only time will tell how the iPhone will fare as a gaming platform. Unlike Sony, Nintento, and Microsoft, Apple does not have any in-house game development. So, since the success or failure of a system really depends on the games more than the system, things will rest in the hands of developers and consumers come June. But if I were Nintendo, the current reigning champ of the portable gaming market, I'd be watching Apple very carefully -- and working feverishly to develop the next generation of my portable hardware.