It's simple: those who want to, can follow their rules and those who don't want to, need not follow their rules. Besides there are enough tools available to hide your "illegal" use of Bitcoin that I think no one will ever follow any rules other than the agreed upon Bitcoin rules they have to follow if they want to use it.
I said assume it catches on and replaces PayPal or Visa. At this point governments will regulate it, and they *may* do this by adjusting the rules of the network. For example they may publish a blacklist of known money laundering accounts and say that a valid block must not contain transactions from accounts in that blacklist, or maybe they'll go for a whitelist approach and force people to register their addresses for tax reasons, maybe they'll put some central override in for returning stolen coins. Whatever those rule changes it will create a regional fork the block-chain, legitimate merchants in the affected region won't be able to accept vanilla BTC and this will fuck with the economy.