He gave this graph to show that temperatures in Salem, Oregon were in fact decreasing:
these data via Mark's site.
However, what if we look at the daily average temperature, defined as the average of Tmax and Tmin for the day? The result is quite different, with a positive trend:
Anthropogenic global warming increases nightly minima more than it does daily maxima. So just looking at the trend in daily maxima can be misleading. Better to look at the trend in the daily average.
(note 2/13: corrected the label on the last graph.)