The iPhone app store, made possible with the release of the iPhone software developer kit (SDK) earlier this year, launched this week to rave reviews across the Web. The store and its open approach to applications heralds major changes in the mobile world.

The store, which is available not only to new iPhone 2.0 users but via software upgrade to existing iPhone customers, launched with just over 500 applications. According to analysis by iPhone tools developer Pinch Media, 70% of the available apps cost less than $5, with 135 apps free.

Available apps ranged from VoIP over WiFi (Truphone); Web clients (AIM, Twitterific); location-aware apps (Yelp, Loopt); social networks (MySpace, Facebook, Shozu); multimedia (Pandora, Midomi); entertainment and games (Sega's Super Monkey Ball, MLB.com At Bat, Cro-Mag Rally); and utilities (iTunes Remote).

Here’s why the iPhone App Store matters:

Slick Interface, Flawless Integration, Billing Matters

Not surprisingly, the iPhone App Store looks great. Even more impressive is its seamless integration into the phone itself and, more importantly, into iTunes running on a user’s computer. Apps can be purchased at either “location” and synched between the two. One of the great advantages of incumbent wireless carriers is their billing relationships with customers. While AT&T (and other iPhone carrier partners) continue to maintain those primary billing relationships, the app store/iPhone combo enhances Apple’s already strong ability to reach into its customers’ pockets.