Going back through my records, I see a 6% yield hit for second-year corn. When I did the study (a couple years ago) I didn't have enough third-year corn to be statistically meaningful, but the few observations I did have looked it was about the same as second-year yields. I should update this, since we've gone from a predominantly c-s rotation to c-c-s. With the occasional extra "c" thrown in, I may have a number I can hang my hat on by now.

The reason usually given for depressed yields in corn-after-corn is allelopathy. Something to keep in mind as we look at 2008 prices and all those 2007 corn stalks....

http://www.ipm.iastate.edu/ipm/icm/2007/2-12/allelopathy.html