Studios like PopCap and EA have been developing mobile phone games for years, but they’ve never seen anything quite like Apple’s iPhone. The device’s touch screen, motion sensors and high-definition graphics open up intriguing new possibilities for game design – and companies have already starting to take advantage of them.

The new iPhone 3G, which launches today in 21 countries, features several improvements over the original iPhone, but the most important one for gamers is its ability to download and play more than 150 games from the brand new Apple App Store.

The App Store is similar to Apple’s iTunes music service. By surfing to the site on your iPhone, you can purchase and download a variety of applications directly to the phone. According to Apple, there are more than 500 applications in the store, and about one third of those are games.

Several well-known studios have released iPhone games so far, including PopCap (Bejeweled 2), EA (Scrabble, EA Sudoku and Tetris), I-play (Numba – a brand new property for the iPhone) and Namco (Pac-Man and Ms. Pac-Man). Freeze Tag has released a version of its PC download game, Etch A Sketch, for the iPhone as well.

Developers seem most excited about the unique things they can do with the iPhone that opens up new ways for players to interact with games. In Scrabble, for example, players can use the touch screen to zoom in and out of the board and drag and drop tiles, and can shuffle the tiles simply by giving the phone a shake. In Numba, players make matches by dragging their finger across the screen, and if they run out of matches they can shake the phone to jumble the puzzle board and create new combinations. Etch a Sketch lets you shake the phone to erase your doodles.

Etch A Sketch on iPhone is a doodler’s dream,” said Craig Holland, CEO of Freeze Tag. “With the innovative multi-touch user interface you can actually create artwork by touching both ‘digital’ knobs at the same time, something you can’t do on any other wireless device, or you can just use your fingers to draw directly on the gray canvas.”

At the time this article was written, the Top 10 downloads on the App Store were all games, with the number one download being an iPhone version of Sega’s popular console game Super Monkey Ball.

“Apple’s iPhone and iPod touch open up the world of games to an entirely new audience,” said David Cobb, VP of Product Development at Sega. “The touch and tilt controls are natural and easy to learn, making gaming even more inviting, creative and rewarding. Super Monkey Ball takes advantage of these defining aspects and offers consumers an extremely accessible, imaginative and fun game experience.”

Games are being sold for between $4.99 and $9.99 each. Click here to check out the App Store.