This post is a kind of work in progress, as much info I get that make me change my mind as much the "rules" are changed. I'm not after imposing my opinion, just to see whether there is an issue or not. So I "changed my rules" because of the previous post. But thank you for many valid points explained.
If you are trying to find out if there are ways that gullible people can be scammed, history tells us that the answer is always YES.
If you are trying to determine if a savvy user taking the proper precautions can be scammed, the answer is: Maybe. It depends on motivations, risk tolerance and awareness, and many other factors.
If you are trying to determine if Bitcoin-Qt itself has a fatal flaw that someone could take advantage of, the answer is: Yes. Bitcoin-Qt assumes a "rational actor". If someone (or a group of people) is willing to spend huge amounts of money to turn another person (or group of people) into victims, without gaining any monetary benefit for themselves, then a it would technically be possible to acquire more hashpower than the rest of the world combined and create havoc with anyone they want to.
Seems like performing this kind of attack against a lite wallet is much easier, right?
Performing an attack that takes advantage of the victim's trust is always possible. It doesn't matter what form that trust takes. Using a Bitcoin-Qt that is provided to you by the attacker is a form of trust. Accepting an unconfirmed transaction is a form of trust. Accepting a single confirmation on a BTC transaction valued at $200,000.00 is a form of trust. Using a light wallet is a form of trust. Using a hosted wallet is a form of trust. Using a block explorer is a form of trust. Using paypal is a form of trust. Using paper checks from a bank is a form of trust.
Choose your favorite form of trust, assign it to the potential victim, and yes, it will be possible to take advantage of that trust.