Apple just finished their special event. They announced new, faster, and bigger iPod Touches, colorful iPod Nanos with a video camera, a new generation of the tiny Shuffle, and iTunes 9 with a bunch of nice new features, and iPhone OS 3.1 (I probably missed a few things there, but those are the main ones).
But the big news for developers is that there are now over 50 million iPhone OS devices in existence. Admittedly, some have been decommissioned, but any way you look at it, that's a lot of devices for a platform that's only been around a little over two years.
The breakdown, according to Steve is that they've sold more than 30 million iPhones and over 20 million iPod Touches.
To put that in perspective, it's greater than the installed based of all Mac OS X devices. Installed base is always an estimate, but the estimates I've seen put the Mac installed base at between 30 and 40 million devices.
Yes, the app review process can be a pain. The seemingly arbitrary rejections suck when they happen, and the expected low price point can be problematic. But, there really isn't a close second. There isn't a viable alternative. There's no single place to reach this many mobile users this easily, even in those situations where it's not necessarily that easy.