Here is the specific rule just posted on the TSA website
Public Notice
For Travelers
Passengers may observe additional security measures related to insulated beverage containers. TSA is carefully monitoring information related to terrorist tactics and working with our international partners to share information and security best practices.
The possible tactics terrorists might use include the concealment of explosives inside insulated beverage containers, so in the coming days, passengers flying within and to the U.S. may notice additional security measures related to insulated beverage containers. While such items are not being banned from travel, TSA Officers have been trained to detect a variety of threats including the concealment of explosives in common items.
TSA will continue to deploy risk-based security measures and work with our international, federal, state, local and private sector partners to protect the traveling public.
As always, the safety and security of the American people is our highest priority and we ask the public to remain vigilant and aware of their surroundings and report any suspicious activity to their local authorities.
Reading further in the Q&A for this notice indicated that this only applies to insulated beverage containers.