The average 2010 CO2 concentration to date is 390.21ppm, based on data found here.
The average for 2010 will not be above 390ppm since there are three months left, and those three months have historically had concentrations slightly below or a little above the September measurement. In 2010’s case, that measurement was 386.80ppm. Using low estimates for the next three months, I think 2010’s average CO2 concentration will be closer to 389ppm.
Since we continue to spew billions of tons of CO2 into the atmosphere every year, this will be the last year for quite some time that the yearly average measured below 390ppm.