Will third-party programs push Apple's handheld over the top?

One program did strike me as being truly revolutionary and worth every hassle you go through just to get an iPhone these days: Pandora radio. You may know Pandora from its many years on the Web; the much-beloved Internet radio service analyzes your musical taste according to a rigorous music-theory system and offers up songs it thinks you might like. Often, it's frighteningly accurate: Play Pandora on your computer for a few hours, and you start to suspect the software's tapped into your frontal lobes.

Like other Internet radio stations, Pandora has been trying to offer a mobile service for many years but hadn't had much luck convincing cellular carriers to play along. Last year, AT&T and Sprint signed up with Pandora, but both carriers required a monthly fee ($9 on AT&T and $3 on Sprint). Tim Westergren, the composer who founded Pandora, says that he's always wanted to offer a free, ad-supported streaming mobile radio station. The new iPhone lets him do just that.

When you load up Pandora, music that you love—some songs you know, some you don't—streams from the Internet and into your earbuds at no charge. You can tell the app what you like by pressing a thumbs-up or -down button, and you can always skip a track. (Buying songs you like is also very easy.) Do this for a few minutes, and you begin to wonder how you'll ever live without it.

Streaming radio is the perfect complement to an MP3 player—you could say it's the part of the iPod that's been missing all along. While your iPod plays music you already own, Pandora gives you music you don't know but will probably like. Now, with the iPhone and its built-in iPod, you've got both: a full library of songs you've purchased and the entire Internet of songs you might like.

Not everyone is happy with the new App Store system. The iPhone, to the consternation of some developers, is still not completely "open" to outside apps—at least not in the way your desktop machine is. Apple takes a cut of every app sold, for example, and imposes its own user-interface guidelines and technical limitations that some say are meant to stifle competition.

But the doors may have been opened just far enough to change everything. Since the store launched, iPhone users have streamed 3 million tracks through Pandora, and Westergren says they're listening for an average of nearly an hour a day. It's been up for less than a week, and the iPhone App Store has already killed Top 40 radio. What's next?