Comparing the number of tornadoes in a given year against historical values is difficult to do. From data available in the 'SPC observed tornado dataset' it would seem that there is a steady increase in the annual number of tornadoes in the US:
Yet, this dataset is far from homogenous. Population increase, increased tornado awareness, and more robust and advanced reporting networks have lead to an inflation of the number of tornadoes being reported each year. In order to be able to compare the annual tornado counts SPC created the 'Inflation Adjusted Tornado Trend' which is shown in the plot below:
This plot does not use the actual number of reported tornadoes in a year, but an estimate of how many tornadoes would have been reported in 2007 if the actual number of tornadoes for that year would have occured in 2007.
What you can see in the plot are the minimum and maximum (adjusted) tornado counts for each date in a year (as well as the 25, 50 and 75 percentiles, but we'll ignore them for now).
The black line represents the number of tornado reports in the current year, in this case up to May 7. This number is based upon the Local Storm Reports, which do contain duplicate reports. The actual number of tornadoes after analysis of the data usually turns out to be about 15% lower, so the number of reports has been multiplied by 0.85.
Looking at the black line up to May 7th, we see that after the Leap Day outbreak, 2012 briefly flirted with the maximum numbers of tornadoes. (Which prompted Accuweather to publish this article: Above-Normal Number of Tornadoes Expected in 2012) And indeed, for two brief periods in March and April 2012 actually did become the year with the most tornadoes.
Now let's look at the most recent plot for this year:
About a week ago 2012 has become the year with the fewest number of tornadoes!
With 5 weeks left in the year, 2012 still has some time to catch up, but 2012 certainly could become a historical tornado year.
(Source: Storm Prediction Center WCM Page)