Preorders for the next-generation iPhone, the iPhone 4S, have begun at Verizon Wireless, AT&T and Sprint Nextel. The new 4S, which was unveiled Tuesday at a high-profile event in Cupertino, Calif., at Apple headquarters, will ship Oct. 14.

The iPhone 4S will be available for $199.99 (16GB), $299.99 (32GB) and $399.99 (64GB) with two-year contract. In addition, AT&T will offer the iPhone 4 at $99.99 for a new 8GB model.

To hasten the process of upgrading for customers, AT&T has rolled out the ATT Upgrader app for iOS devices. The app asks customers for some basic information and then alerts them to their upgrade status, allowing them to order the new phone directly from their existing devices.

In a statement released today, AT&T acknowledged past snafus with the ordering process for the iPhone. Demand for the iPhone 4 was so great that both Apple and AT&T servers went down. “Our past experience with iPhone has taught us that speed is the name of the game,” said Ralph de la Vega, president and CEO of AT&T Mobility and Consumer Markets, in a statement. “The faster we help our customers, the better. The faster we ship their new device, the better. And – the faster their network – even better.”

Verizon Wireless and Sprint Nextel are also both accepting preorders for the new phone. Sprint yesterday announced that it will offer unlimited data plans with the iPhone, in an attempt to distinguish itself from rivals AT&T and Verizon Wireless, which have phased out their unlimited data plans for the devices.