T-Mobile is offering Apple iPhone users up to $350 for trading in their smartphone for an HTC HD2, a competing device powered by Microsoft Windows Mobile 6.5.

The wireless carrier confirmed the promotion in an e-mail sent late Monday to InformationWeek. The trade-in offer runs through May 19.

How much an iPhone user receives will vary based on the generation and memory of the device. However, T-Mobile said it is willing to offer as much as $350 to get the Apple customer to switch.

T-Mobile offers the HD2 for $200 with a two-year service plan. If the buyer prefers to skip the long-term commitment and pay month-to-month, than the phone costs $450.

The HD2 received generally strong reviews when it was introduced in October last year. While reviewers were critical of Windows Mobile 6.5, HTC bypassed the shortcomings with its own Sense interface.

The device packs a 4.3-inch capacitive touch screen that's much larger than the screen on the iPhone and most other touch devices. Inside, the device packs a 1-GHz Snapdragon-based processor, which provides plenty of horsepower to multitask. The smartphone also includes Wi-Fi, GPS, 3G, a 5-megapixel camera, and a standard headphone jack.

The major drawback to the HD2 comes from Microsoft. People who buy the device will not be able to upgrade to Windows Phone 7, which Microsoft introduced in February. From the user's standpoint, the new OS, which will be available this year, shares only the name "Windows" with previous versions of Microsoft's mobile operating system, according to reviews.

Most notably, Windows Phone 7 features a start screen that trades static icons in favor of real-time feeds from the Internet's most popular social-networking sites, including Facebook and Twitter, as well as other applications. It also offers always-on access to e-mail, Xbox games, Bing search, and other tools. The platform also includes a music and video player.

All the major improvements of the new OS won't be available on the HD2. In abandoning backward compatibility, Microsoft is starting fresh in the smartphone market, where it lags far behind Apple and Research in Motion, which makes the BlackBerry.

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