ComScore is out with a new report this week regarding US smartphone platform marketshare for the March quarter. And the highly-regarded marketing research firm provides an interesting look at the current smartphone landscape by OS.

Google, as you might have expected, is still in the lead with its Android operating system. Apple’s iOS is in a [comparatively] close second, and RIM and Microsoft come in at a distant third and fourth. Keep reading for a full breakdown…

According to comScore’s MobiLens service, Android currently owns 51.0% of the U.S. smartphone market, up from 47.3% in the December quarter. iOS is also up from the previous quarter, sitting just behind Android with 30.7% of the market.

Then there’s RIM, in a distant third place, with just 12.3% marketshare. The Canadian company has actually dropped 3.7% since the December rankings, as it continues to bleed marketshare. And Microsoft’s platform, Windows Phone 7, comes in dead last at 3.9%.

We talked about this a bit yesterday after seeing RIM unveil its new BlackBerry 10 OS. The smartphone platform market has become a two-horse race between Android and iOS. But who is going to to snag, and more importantly hold on to, that crucial third spot?

Microsoft or RIM?