And so what if it is 'declared' illegal? Who is going to declare this? By doing so, they are admitting their fear of bitcoin and it will only strengthen the cause. By making it illegal, they are reinforcing the ideals that drove the creation of the technology in the first place.
Bitcoin is anonymous money which have no smell. This is what US Gov fights against. In 2000 there was a lot of anonymous banks which ditributed anonymous cards (Parex bank in Latvia, lots of cyprus banks, etc). You could send and receive almost any amounts all over the world, cash it and nobody never asked where the money from.
After 9/11 US declared holy war against "terrorism" and started to destroy any banks with pseudo-anonymous customers, processing black and gray money.
Can you get anonymous (without documents) VISA/MC today? Show me the places.
So once bitcoin turnover will grow above some visible limit, it will be declared as illegal, facilitating drugs/weapons/terrorism activities.
And who uses bitcoin now except Silk Road, HYIPs & gamblings? Nobody. While black market has abilities to sold their stuff bypassing government control.
This will have effects in a future.