A peer-reviewed paper published in the Journal of Atmospheric and Solar-Terrestrial Physics (de Jager, C. and Duhau, S. 2009. Episodes of relative global warming. Journal of Atmospheric and Solar-Terrestrial Physics 71: 194-198. ) finds global warming over the 20th century "not significantly different" from warming episodes that occurred in earlier centuries. The paper finds that the increase in solar activity over the past 400 years explains the warming, without any need to search for a unique cause of late 20th century warming, such as greenhouse gas concentrations.
The NIPCC has posted a summary of the paper here.
Hockey Stick via Marc Morano.