As RTSmith said regarding Tennesse, in Idaho, if the GCVWR is 26,000 pounds or over, then the CDL is required long with the DOT number etc. The distinction is that it does not matter if your gross weight is under 26k, if the weight rating of the power unit and trailer exceeds 26000 you are subject to those rules.
Edited afterthought - What sucks is that technically when we unhook the trailer from the pickup we are still supposed to have a CDL (as we are licensed for 30,000 lbs.) to drive just the pickup, which in my case is also my personal pickup. This means that my wife is not supposed to drive my pickup because she does not possess a CDL. We don't pay much attention to that though, and have not had any problems when just the pickup is being driven.