These data spell significant trouble for Microsoft in the smartphone market. Despite launching Windows Phone 7 (WP7) with great fanfare in November 2010, Microsoft's market share in smartphones has in fact declined 1.7% from October 2010 through January 2011.
They weren't many phones supporting WP7 until recently. I think you will see that number grow slowly. They're a bit late to the party. I think it's a better OS than Android in my view. That said, the only people I know with WP7 are .NET developers ;)
That's not what the term means, though, and using the real definition of market share it is impossible to have a negative market share, for the exact same reason that you can't have a negative number of people using a product. "Small market share and spectacularly unsuccessful products" is not, even in the loosest possible sense, the same as "negative market share."
The market is growing, so it's not as simple as that. It's more like RIM is capturing below market rate from the new smartphone users, Apple is almost exactly at market rate and Android's scooping up the rest.
How people move between platforms would be interesting, and become more important as smartphones reach saturation, but you can't really speculate from this data.
