[bearmtnmartin]

I always thought it was based on brakes and driveline strength

http://www.canadiandriver.com/2009/08/12/auto-tech-towing-limits.htm

 

[PTSolutions]

i guess that will be changing soon:

Maximum trailering is up to 20,000 pounds pulling a fifth wheel trailer with a DRW Duramax diesel and up to 14,500 pounds pulling a fifth wheel trailer with a SRW 6.0-liter gas V-8. GM says the towing specs were calculated using SAE's new industry J-2807 trailer towing standard that will eventually be followed by all truck manufacturers.

that was taken from a look over of the new 11' chevy trucks, so it seems that there will be an industry wide standard of establishing tow ratings.