The TSA specifically allows tripods in carry on luggage per their website:
Tripods
Carry On Bags: Yes
Checked Bags: Yes
Tripods
I'm going to print out that page and put it in my carry on bag, just in case there is a question about it.
Note that even at the bottom of that page, it does say, "The final decision rests with the TSA officer on whether an item is allowed through the checkpoint."