It can be used for measuring both the atmospheric pressure as well as its temperature. From the web page:
"The pressure value from the sensor is in 1/1000th of inch Mercury (inHg) and the temperature is in 1/10th degree Celsius. The NXT-G block allows the NXT to read the pressure in either inHg or hPa (hectopascals) and temperature in either Celsius or Fahrenheit degrees. Once adjusted for current conditions, the block can also calculate altitude in feet or meters."As usual, HiTechnic also provides support for the new sensor in NXT-G as well as in RobotC.
By experience I'm positive that other NXT platforms like leJOS NXJ or NXC will integrate support soon, too.