People are starting to look seriously at the iPhone 4 as a robotics controller. It's got a full IMU and otherwise looks appealing. You can get all the hardware chip and sensor specs from this excellent iFixIt teardown, but here's just one example: the GPS chip.
It's a Broadcom chip and the refresh rate is "up to 2Hz". The user-accessible data is provided by the OS so the refresh rate in practice may vary, but the hardware looks pretty promising.
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I believe in the use of an arduino as an interface between the real world (sensors) and the big brain (that may be a smartphone).
As for what smartphone will be the most used, I guess it would be an open one with usb-host support for communication.