The best current value now being released by the NASA COBE Far Infrared Absolute Spectrophotometer (FIRAS) experiment, based on all 9 months of the available observations is:
T = 2.728 +/- 0.002 Kelvins
The uncertainty that has previously dominated the statistics was caused by a variety of systematic errors in the calibration and analysis procedures which have now been substantially reduced. The first measurement published in 1990 of T = 2.735 +/- 0.060 K was based on only 9 minutes of data.