AT&T previously said it was planning its biggest launch ever for the just-released Nokia Lumia 900, even bigger than the iPhone. Now we have a figure that shows AT&T really is putting up serious dough: $150 million, according to Ad Age.

Oddly, AT&T launched the Lumia 900 yesterday on Easter Sunday, when many AT&T stores were closed. But the company is looking to push this phone heavily for a few months, with up to $150 million dedicated to promoting the device. Ad Age notes that AT&T spent $3 billion on advertising in 2010, so $150 million on a single campaign won’t hurt its bottom line much.

The Lumia 900 has the best chance of selling Americans on Microsoft’s still-young Windows Phone 7 OS. VentureBeat’s own Devindra Hardawar is bullish on the Lumia 900, calling it the best $100 phone on the market. The device features a huge 4.3-inch screen, LTE 4G connectivity, an 8-megapixel rear camera, a front-facing camera, and an updated version of Windows Phone Mango (7.5).

Thus far, the biggest drawback for consumers using the Windows Phone OS instead of iOS or Android is apps. While there are 70,000 apps or so in the Windows Marketplace, the competition has many more, as well as more recognizable titles. To remedy that, Microsoft is writing checks to developers to help its app store catch up.

Lumia 900 image: Sean Ludwig/VentureBeat