TSA has conducted a risk assessment that basically says short, thin folding knife blades are an acceptable risk at this time. They may have a better way of saying it on the TSA.gov web site.
The risk assessment still bans locking blade knives and a few other restrictions, like box cutters and such. This piece will not go into the risk finding regarding golf clubs and baseball bats. The following picture is of a knife similar to the one found in the field in Pennsylvania from the 911 attacks. Like the one found it is a lock blade knife that also is a cigar lighter, the first shot is of it closed:
My issue is not with the risk analysis that said the knives they are going to allow are an acceptable risk, my issue is with why the locking blade knives and molded handle knives are not being considered risk acceptable. I can carry my briefcase on board, and have been for years, it is stiffened by a long thin very sharp piece of metal, that comes out of the case with out any problem at all. There are so many items in the cabin that can be made into knives or sharp bladed objects, by any one that has ever been in prison,or watches reality prison TV.
Accepting risk is actually at times good security, and I believe that TSA as far as they went made a good risk assessment call. But stay true to your risk assessment.