Oh please, that's easy. First you start by talking 12-digit codes with Nintendo and then pinky-swear that there won't be any guitar playing pedophiles on the other end of that game -- to prove this each purchaser of Guitar Hero III who wants to go online will submit two forms of ID and authorize a criminal background check by Nintendo. It's as simple as that.
Update: Red Octane contacted Joystiq to say that Huang was talking about downloadable content to ProG, not online play. There will be no DLC initially for the Wii, but they reaffirm that there will be online play. Of course, our first question was, "How are you implementing that?" They have to check with the developer and get back to us.