Microsoft's new smartphone platform is off to what could be considered a slower start than expected in North America. That's according to The Street, which has released a report saying that the company sold some 40,000 units on its first day on the market.

That number does not come from Microsoft, which has not yet released sales numbers and refused to comment to both The Street and CNET, but rather a source in a market research firm.

Windows Phone 7, which is the follow-up to Microsoft's Windows Mobile 6.5 platform, hit shelves in the U.S. and Canada on Monday. Before that, WP7 devices had come to stores in Europe, Australia, and New Zealand in late October.

CNET hoofed it to two major carrier stores in San Francisco on launch day and came back with the impression that both the inventory and sales of the launch devices had been quite low, though the scene had been different in other cities and retail locations, where some stores were selling out.

Early sales numbers from other phone platform launches include Apple's estimated 500,000 iPhones being snatched up during its launch weekend in 2007, and a million and a half G1 Android phones (the first Android device on the market) being bought up by T-Mobile subscribers in the phone's first six months. Those numbers have since improved with later device launches on the same platform such as the iPhone 3G and 3GS which both hit a million units in their first weekend on sale, as well as the iPhone 4, which sold 1.7 million units in its first weekend. Google too recently pegged Android activations as being north of 200,000 a day.

Despite the reported low sales, reviews of many of the launch devices, as well as the Windows Phone 7 OS itself have been quite positive.