All i have to say is Liberty Reserve...same fate for Ripple.
Completely different situation.
Opencoin operates in the US under US law.
Opencoin is in the US and under US law, thus it will be even easier for the US to shut them down than it was for LR. Look at how easy it is to open an anonymous ripple account: they don't even ask you for a name and email address.
Pretty much this, especially since Ripple seems to be promoted for facilitating real money transactions. The debt/finance aspect probably opens up new legal vulnerabilities even Liberty Reserve didn't get into.
On the otherhand bitcoin is simply an online game where the developer neglected to build anything other than the currency.