Well, according to NOAA's National Climatic Data Center, 2006 was even warmer than 2005. That means, if you haven't been keeping score, that 2006 was the warmest year on record (for the USA). I didn't find an actual ranking for the global mean temperature, but I think we can safely assume 2006 was in the top 5 (if not the top 2). The press release blames (rightly, for a change) ENSO (which is El Nino) for the getting 2006 into the top seat. It turns out December was really hot, mostly because there weren't very many storms crossing the country. My suspicion is that this El Nino will continue to make this winter much warmer than average, and 2007 will beat 1998 and 2006 as the warmest year in the past 1000+ years.
LINK TO NCDC