The biggest problem is not what is or is not permitted, but the consistancy with which the rules are applied and the over zealous approach taken by some so called security agents.
I have had many occasions of having items be examined and passed, only to have them rejected for my return flight the same day. In all cases, these were items I could have bought having passed through security. Fortunately I live in a country where questioning back politely pointing this out won't get you locked up and in all except one case, the supervisor permitted me to keep the item.
Items include a laptop computer security cable and spare stylus for a PDA.
As many have alluded to, the only sure way to remove all items of risk is to provide all passengers "cabin clothing", conduct a full cavity search and permit no luggage in the cabin. That would obviously result in no one flying, so a ballanced approach has to be taken - weigh up risk against economics - after all, isn't that exactly what we all do when we cross the road, drive a car, or fly as a pilot or passenger.
Unfortunately, decent threat profiling doesn't seem to be permitted due to civil liberties groups in the US, so as a result ALL have their civil liberties removed, causing delays and a desire to avoid travelling to the US if possible.