I'm, like sure this is out there somewhere already, but if not, it should be:

The U.S. Constitution, online in sections/paragraphs, etc, so that each bit can be the start of a discussion thread or the target of annotion.

There's a (possibly useful) set of Supreme Court annotations here.

Here's a good one, The Founder's Constitution. No discussion, but it looks like they have some sort of annotations – it's not obvious to me what they refer to.

Another – FindLaw: Cases and Codes: U.S. Constitution. Searchable and annotated, but no public discussion. That's important to me.

The U.S. Consitution Online. Not bad, but again… no public discussion.

I do like that the Founder's Consitution has permalinks. Is it really that important to have public discussion? I don't know. It' seems so to me – or at least, it seems that there should be at least one online copy out there that does.

I did not go past one page at Google – I was just curious. If anyone already knows about a site that offeres public discussion on specific seelections from the U.S. Constitution, I'd love to hear about it.