$1 billion is unreasonable. The USD monetary base is about $4 trillion ($800 billion before Bernanke went on his drunken money printing spree). So, if BTC completely replaced USD, 1 bitcoin would be worth $4 trillion divided by 21 million, or $190,000 and that is no where near $1 billion.
I have to agree. Even if one Bitcoin was one billion, some currency bans and heavy taxing would occur.