The peer-reviewed version of the analysis of trends on spending in training was just published in Performance Improvement Quarterly.
As noted in the article, after adjusting for inflation, total spending has only grown 1.5 %, even though the US workforce grew by more than 30%.
Here's the full citation, in case you want to track down the article:
Carliner, S. & Bakir, I. Trends in spending on training: An analysis of the 1982 through 2008 Training Annual Industry Reports. Performance Improvement Quarterly, 2 3 ( 3 ), 77–105.