You're probably right, Greg. I think there has been a standing invitation for the President to speak for as long as there have been National Jamborees. FDR attended the first National Jamboree in 1937. It had been scheduled to celebrate the 25th anniversary of Scouting in 1935, but had to be postponed because of an outbreak of polio.
My brother-in-law and my nephews went to at least one Jamboree, but I never got the chance.
-Joe-