An international working group called INTCAL has announced an updated radiocarbon calibration curve. As Michael Balter notes in ScienceNOW online:
To calibrate the period extending from the present to about 12,000 years ago, the team has used thousands of overlapping tree-ring segments from the Northern Hemisphere, which provide a very accurate check of raw radiocarbon dates and how much they must be corrected. But for dates older than the available tree-ring record, the researchers had to turn to several other, less-precise data sets on ancient CO2 levels, including fossil foraminifers (single-celled organisms that secrete calcium carbonate) and corals.
The new curve is called IntCal09, and is available here.
The technical article, published in the December 2009 issue of the journal Radiocarbon, is here (lead author is Reimer), but is not free.
Reimer, Paula J., Mike G.L. Baillie, E. Bard, Alex Bayliss, J. Warren Beck, Paul G. Blackwell, Christopher Bronk Ramsey, Caitlin E. Buck, G.S. Burr, R. Lawrence Edwards, Michael Friedrich, Pieter M. Grootes, Thomas P. Guilderson, Irka Hajdas, T.J. Heaton, Alan G. Hogg, Konrad A. Hughen, Klaus Felix Kaiser, Bernd Kromer, F.G. McCormac, Sturt W. Manning, Ron W. Reimer, D.A. Richards, J.R. Southon, Sahra Talamo, Chris S.M. Turney, Johannes van der Plicht, and Constanze E. Weyhenmeyer. 2009. IntCal09 and Marine09 Radiocarbon Age Calibration Curves, 0–50,000 Years cal BP. Radiocarbon 51:1111–50.