April 8 is going to be a big day for Microsoft (Nasdaq: MSFT).

It's the day when AT&T (NYSE: T) will begin selling Nokia's (NYSE: NOK) Lumia 900.

If that doesn't seem like an important event for the world's largest software company, consider that this will be Microsoft's best chance to matter in a world where Apple (Nasdaq: AAPL) and Google's (Nasdaq: GOOG) Android appear to be running away with the smartphone market.

Nokia's flagship phone is powered by Microsoft's fledgling Windows Phone operating system. Microsoft is so adamant on making its mobile platform stick that it has struck a deal with Nokia that will reportedly send billions to the Finnish handset maker in exchange for rallying around Windows Phone.

Microsoft and Nokia are joined at the hip here. Now they just hope that that Lumia 900 is hip here.

Windows to the world
There have been a few earlier Lumia models, but this is really the model that it will all come down to. Nokia is packing plenty of specs into the smartphone, and by landing a subsidy through AT&T as the exclusive carrier folks will be able to buy the high-end 4G LTE device for just $99.99 with a two-year contract.

There's plenty riding on this, and Nokia and Microsoft are hoping AT&T will lead by example. Reports indicate that Nokia is paying $25 million just to give AT&T employees Lumia 900 handsets as their company-issued smartphone. The plan is for employees to be well-educated about the product. The obvious in-store visibility is a sweet bonus.

The rub here is that it will ultimately be the consumers who decide. If they're too resistant to the tiled layout of Windows Phone or if the bulging app store marketplaces at Apple and Android are too much to overcome, it won't matter that the Lumia comes at a great price with sweet specs and a strong marketing push.

Gunning for the bronze
Microsoft and Nokia know that they're unlikely to catch up to Apple and Android as the smartphone platform of choice. However, Research In Motion's BlackBerry is certainly vulnerable. If Microsoft is even mildly successful here, it may not be long before it overtakes a meandering BlackBerry for the third-place slot.

Settling for bronze would be an odd thing for two companies that are used to being on top. Microsoft remains the world's biggest software company, and Nokia is the undisputed global leader in handsets (though not smartphones).

Nokia and Microsoft are calling. Will stateside shoppers be brave enough to answer? The question will begin to be tackled on April 8.

Motley Fool contributor Rick Munarriz does not own shares in any stocks in this article. The Motley Fool owns shares of Apple, Google, and Microsoft. Motley Fool newsletter services have recommended buying shares of Google, Nokia, Apple, and Microsoft. Motley Fool newsletter services have recommended creating a bull call spread position in Microsoft. Motley Fool newsletter services have recommended creating a bull call spread position in Apple.