A Litecoin ASIC is possible, if a billionaire wanted to build enough ASIC for both Bitcoin and Litecoin 51% attacks they could. Be careful thinking that Litecoin is immune to ASIC, its immune to Bitcoin ASIC of course.
How much money did it cost to build a ASIC? A order of magnitude cheaper than Intel spends on its new Fabs.
Avalon and ASICminer designed and created ASIC's for under $50 million.
I'm not sure about the situation here, with ASIC in general existing and being so much more powerful than conventional chips.
A superior long term crypto-currency will one that requires a exponential amount of power to attack the system. The fact that Bitcoin and Litecoin attack vectors require a liner increase in power in regards to current network power presents a problem.
Even though there are ways to recover from a 51% attack, A bad actor could sustain the attack until confidence in the currency was destroyed, remember the only real reason anything has value is because of confidence.
You have confidence in Gold and that people are quite likely to always buy it from you, you might have less confidence in USD but at least the USA will honour it (This is why a government default is so dangerous, destruction of confidence in a nations currency would destroy that government or even the nation itself) and Bitcoin and Litecoin have value because the people have decided they have value. I believe that crypto-currency confidence, even though it can cause bubbles, is the one of the most powerful types of confidence, a type of network confidence)
However if someone sustained a 51% attack against Litecoin and Bitcoin, it would kill this confidence.
If a solution is found, a network that requires a exponential attack to gain enough power, then either a competing currency would use this and overtake Bitcoin, or Bitcoin would adopt it as a superior system of security.
This is the true holy grail of crypto-currencies, if you are very smart and reading this, I beg you to help find a solution.
Take the attack against the wallet address, it requires so much computation that it is not feasible. If the network was that difficult to attack, then we have winner. 51% is too easy, until it requires a hundred thousands ASIC devices running the network. Then we might be able to say its a little secure.
Don't believe me? Run the numbers, ASIC exist and they cost no where near a billion to make. Think about it.