To date, iPhone games have been largely rubbish. That's not to say the system is without potential, though. Especially when you consider there are over 30 million of them in the wild.
Announced during an Apple event yesterday, that figure includes both iPhones and the new iPod Touch, which both can run games (and other apps) from the App Store. It's a big, big number. Not as instantly significant as console sales numbers, true (since there will be people who don't game on their iPhone), but still significant.
Why? Because, unlike the generic term "mobile phones", we're not talking an immeasurable number of different devices with different capabilities spread across different companies. We're talking two devices, from the same company, which both draw their games from the same, single location.
That streamlines development. It centralises the marketplace. Which is appealing to developers, which results in more games on the service (yes, more than the 6000 currently available), which means more people playing them.