Will one get in trouble for taking a jailbroken device through airport security?
Solution 1:
From me, a US citizen, I wouldn't worry about it at all.
They shouldn't even touch his iPhone/iPod, let alone turn it on. Also, I can't imagine how he could get in trouble over something that is legal in the US (jailbreaking).
From my experience flying, electronics are just sent through an xray machine. Any search of electronics would make people upset on grounds of the 4th amendment.
Solution 2:
I work for the TSA and no, we cannot see that the device is jailbroken, nor do we care that it is jailbroken...
Solution 3:
In 10 years of flying with some form of smartphone, never once, in any country, including US, has any form of security paid any attention to any device I carried.
If you are concerned, my suggestion is to enable the lock code, lock your phone, and then forget about it. They can't open your phone to look for whatever evidence you think is there.
Solution 4:
As bassplayer said, Jailbreaking is legal in the US (with the exception of the iPad, where the law is rather fuzzy). You should have nothing to worry about.