Last week, the US National Academy of Sciences published a long-awaited report on the "hockey stick". No, the report has nothing to do with the Stanley Cup finals, though feel free to draw a climate change metaphor from the Carolina Hurricanes beating the Edmonton Oilers.
The "hockey stick" is the famous graph depicting global temperatures over the past 1000 years; the blade of the stick is the upswing in temperatures since the Industrial Revolution.

The graph was originally produced from the instrumental temperature record (since 1860) and a collection of temperature proxies -- ice cores, tree rings, etc. -- for the period before widespread temperature measurements are available. It shows that the end of the 20th century was warmer than any other time in the past 1000 years. The authors of the original study also concluded that the 1990s was likely the warmest decade of the millenium, and that 1998 was likely the warmest year of the millenium.

The hockey stick has been the subject of legitimate - and much illegitimate - controversy. Many people skeptical of climate change have criticized, attacked may be more accurate, the methods used to reconstruct temperatures, especially for the period known as the Medieval Warm period (around 1000 AD).

At the request of the federal government, the NAS poured over the data. The report agrees with the general results of the original papers (you can read it all here):

"It can be said with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries... Less confidence can be palced in large-scale surface temperature reconstructions for the period from A.D. 900 AD. Presently available proxy evidence indicates that temperatures at many, but not all, individual locations were higher during the past 25 years than during any period of comparable length since A.D. 900."

The one flaw, I hesitate to use that word since I am not a statistical climatologist, seems to be the argument that the 1990s may have been the warmest decade and that 1998 may have been the warmest year. Both statements are highly likely to be true. However, there is not fine enough resolution in the temperature reconstructions to definitely support such conclusions.