C++ sprintf supports a %f (decimal floating point), but Charles Bell in his book on Sensor Networks says the Arduino version of sprintf does not. Where can one find what format specifiers are useable on the Arduino?
Charles Bell is correct. Floating point output from sprintf() is not implemented by the Arduino IDE environment. This was apparently done to save program space.
You may find this thread and the pages it links to helpful http://forum.arduino.cc/index.php?topic=125946.0