If something is not illegal, as in added to the code of law, it is legal. For bitcoin to become illegal there would have to be established laws against it. If there are no such laws, it's completely legal, and since law doesn't work backwards, if it's ever outlawed, nobody will be able to prosecute you for using it in the past.
Also, it's worth mentioning that some countries already have laws that make bitcoin legal and protected by law, which means if somebody steals your coins you can go to the police, like you would have if they stole physical items.