It's most notably a carrier-influenced result, where Android is available for Verizon, Sprint and T-Mobile, in multiple flavors, with heavy subsidies, including "free" phones. I would expect given the same forward momentum, Android will overtake the RIM OS as number 1 in the US at some point in the future.
Just a couple notes:
1) I don't know of any "free" Android devices.
2) You forgot that AT&T has Android too, though I can understand why. They've insisted on doing horrible things with their one or two Android phones. The Backflip is one of the worst smartphones out there.
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