I've got a bridge to sell you, too.
Social Security Numbers, we were promised, would not be used as personal identifiers. See, e.g., http://query.nytimes.com/gst/fullpage.html?res=9C02EFD71039F...
FOR many years, Social Security cards carried an admonition that they were to be used "for Social Security and tax purposes -- not for identification."
You've got my personal guarantee that if an optional Internet ID is created today, it will be required before long. This is how the system works. Witness the gradual ratcheting up of (ridiculous) security measures in airports if you're skeptical.
If your users post copyrighted material, e.g., then you'll be offered safe harbor only if you enforce registration via the national ID. Same deal for people posting "naughty" content -- which is more insidious than it sounds, because of the overlap between (a) sexual content and privacy concerns in discussions about, e.g., STDs; or (b) in the wake of wikileaks, being hassled about publication of, or support for those publishing, "leaked" data be it from government or from industry.
It's necessary that we provide a way for people to maintain anonymity, but the government wants to offer enough carrots to divert our horse from that path.
I detest these centralized government tracking systems on my activity, and hope that this next attempt on a national ID is quickly defeated.
Cool. They've got the OK from me. It could turn out to be neat, and so long as it's optional I'm not worried.