Hard numbers are hard to find online... all of the industry-related statistics, reports, projections, etc. are only available for a hefty price, but I did manage to find this from the HMA (Hobby Manufacturers Assoc.)
It's interesting that of the major hobby categories, the only one that showed growth was the one that probably has the oldest demographic–model railroading.
Estimated sales numbers 2010 vs. 2012:
Model Railroad 2010: $424,770,000
Model Railroad 2012: $516,465,000
Plastic & Diecast 2010: $305,777,500
Plastic & Diecast 2012: $258,727,500
Radio Control 2010: $362,912,500
Radio Control 2012: $259,477,500
General Hobby 2010: $377,637,500
General Hobby 2012: $272,867,500
Also interesting is that the one segment we generally assume is "healthy" (RC) took a bigger fall during the two year period than Plastic & Diecast did.