The key factor to the success of any currency is liquidity. You are only willing to put money in if you can take money out. If the government of Thailand says it is illegal to transfer money in or out - that will effect liquidity in Thailand. How will this be enforced? First, banks and reputable institutions doing business in Thailand will not be allowed to trade with companies involved in bitcoin exchange. Of course…
Its bad news for bitcoin, but it is very minor bad news. Bitcoin only needs to be legal in the US or Europe for it to work world wide eventually.
Just imagine a time 30 years from now and a company wants to manufacture something in Thailand (lets say it is product as big as the iPhone was in the early 10's.) and that company would like to pay in Bitcoins. Do you think that that government of Thailand is going to keep up their ban?
Of course not, if Bitcoins get big in a few other countries, that is all that matters.
[Edit] Taiwanese replaced with Thailand. Not sure how I made that error.