An average of 392.40ppm CO2 concentration was measured at NOAA’s Mauna Loa, Hawai’i's Observatory during March 2011.
That value is the 3rd highest in recorded history, behind April and May 2010. 392.40 is the highest March value ever recorded. It is 1.39ppm higher than March 2010. It is 0.63ppm higher than February 2011.
A rough extrapolation of last month’s concentration projects out to 394-395ppm in May, the month during which the yearly maximum concentration is recorded.
This will likely be the last year that CO2 concentrations will fall below 390ppm during any calendar month. The aggressive march toward 400ppm continues. Keep in mind that scientists have recommended that 350ppm is the current target for which humanity should aim in order to keep climate extremes from overwhelming our civilization.