Although it’s been up for order for some days via third-parties, the LG Quantum Windows Phone is now officially on the shelves of AT&T Mobility stores. 

The LG Quantum is the fourth Windows Phone 7 powered smartphone available in the US, and the second with a slide-out QWERTY keyboard.  It also has the smallest touchscreen of any of the first crop of Windows Phones at 3.5-inches. There’s no microSD memory expansion, but the Quantum has 16GB of onboard storage, and the rest of the features are pretty much Windows Phone standard: 1GHz CPU, 5MP camera, 802.11b/g/n Wi-Fi, Bluetooth and GPS.

AT&T’s out-the-door price for the LG Quantum is $199 with a two-year contract, but you can grab the phone at Amazon for $0.01 (until midnight)or LetsTalk.com for $99.