here are a couple of blogs about the subject from the TSA board
http://blog.tsa.gov/2009/04/traveling-with-large-amounts-of-cash.html
http://blog.tsa.gov/2009/04/incident-at-st-louis-international.html
heres a post in one of the blogs you can decide for yourself if its BS or not:
There are numerous documented incidents of airline travellers who have been detained and had their cash seized at airports. Even with good documentation (e.g. bank receipts, etc) these folks have had their money seized and been given a receipt. Quite often, it is the airline agent at the ticket counter that tipped off the police.
Good luck trying to get your money back.
As a poker player who travels to Vegas frequently to play in high stakes games, I fly out of LAX with cash amounts ranging from 20 to 200 thousand dollars. I once had 15K in cash taken from me at the checkpoint. My lawyer said it would cost more to fight it, that I should just let it go.
Now if I have to transport more than what will easily fit in the vest pockets of a suit (about 20K in hundreds), I employ an off duty police officer to carry my duffel through the checkpoint. One time I was heading to the Bellagio for the weekend, the bag was heavy with almost half a million in cash. My courier got pulled out of line after the x-ray for further inspection. TSA unzips the bag, the courier flashes his LAPD credentials and the bag gets zipped back up and we proceed on our way.
A round trip ticket and $1000 is cheap insurance.
Recently I flew with my mother on vacation to Hawaii. My mom takes 20 different pills and vitamins daily. She organizes them into a plastic pill holder with compartments labelled for each day of the week. At the security checkpoint the TSA officer threatened my mother with arrest because the pills were not in their original pharmacy bottles with the prescription labels attached. My mother was forced to throw the medicine away (they would not let her to pass through the checkpoint with the 'contraband').
From a policy standpoint, I feel that TSA should not be enforcing any laws or regulations other then those directly associated with aircraft safety. A TSA employee with no police or legal training is not the right person to determine if it is legal to travel with large amounts of cash (it is, by the way), with prescription medications, with pornographic magazines or with t-shirts that have arabic writing on them.