There are only a few ways to even approach the reality of Internet voting, and they all require national ID cards, and all are open to widespread fraud, or at least the possibility of fraud is much greater. It's one thing to lock down voting machines and have them communicate over encrypted connections; it's a completely different thing to enable users to vote from their laptops in their living rooms.
The only secure scenario that I can envision where this would be possible is if everyone had a form of biometric authentication available, whether it's a retina scanner, fingerprint reader, or whatever. Of course, this would require the government to have that information as well, which isn't desirable for a whole host of other reasons. Also, there's no way to verify that the voter isn't being coerced when they cast their ballot, unless there's a video of the event taken during the voting process. Even then, it's not foolproof.
For these and a whole host of other reasons, Internet voting is not viable, and may never be viable. Not surprisingly, this has less to do with technology than human nature.
The middle ground between mechanical systems and Internet voting is electronic voting, and there's no reason why it can't be viable, secure, and foolproof. The fact that this is even an issue in the United States is terribly embarrassing. We are arguably the most technologically advanced society ever to exist on Earth, yet we can't seem to create something so simple, yet so blindingly essential.