Here are current regulations for flights within the U.S.
Note that any agent at any airport can confiscate knitting needles, scissors, etc., at their own discretion. Bamboo or plastic circs seem to be safest; scissors should be the kindergarten type, less than 4" long and with blunt tips. I haven't had any problems in the last couple of years, but but I still put my knitting on a piece of waste yarn before going through security just in case. If you have Denise interchangeables, they're great for traveling.
HOWEVER: none of the above applies to flights outside the US Heathrow Airport in London is especially tough. Check with your airline before traveling outside the US, but remember that regulations can change from day to day.
PS: Some people suggest carrying a self-addressed envelope so you can send your needles home if they don't make it through security. Fine, if you want to go to the back of the line, hunt down a mailbox and wait in line again. The TSA agents will NOT mail them for you.