The idea is the Opencoin will not own Ripple, and that Ripple itself will be decentralized. The system won't be worth anything without gateways, but at the moment I don't see any clear reason why gateways couldn't operate legally. If they comply with the same laws as regular banks do, why not?
Some gateways might, giving the authority less of an excuse, however I suspect that many wouldn't comply. This probably would give them an excuse to try shutting down the entire network.
Of course we hope for the best scenario, and that the resistance of status quo is not too violent. However I think it's a given there will be plenty of bankrupcies of scummy gateways, and excuses to shut the whole thing down would be abundant.
Since anyone can run a rippled server, it would be hard to shut it down. Authorities could attack the gateways, but if a few non-shady gateways start operating that will be pretty difficult too. I'm not sure about the current ones. The location of Bitstamp seems slightly suspect, as if it would be hard to legally establish it in for example the US. WeExchange doesn't even have a stable website (I'm sure the layout is not supposed to look like that!) and is very new.