They're just reinventing history - Tim Berners-Lee's original specifications allowed web-hosted documents to be rewritten on the fly, then WebDAV promised a universal file system based on web protocols. We'll gloss over the PR problems Microsoft had when it introduced public annotation tools for Internet Explorer.
Now we have Google's Blogger Web Comments plug in for Firefox, which does much the same as Microsoft's annotations tools, but this time ties things into their Blogger blogging platform. It compares the URL of the page you're on with links posted on Blogger blogs, and pops up a little floating window showing recent blog postings. It's unfortunate, as Google could have worked with Technorati or PubSub to develop a better database of linking pages - using Blogger only is limiting, considering the number of sites hosted on LJ or Moveable Type or TypePad or WordPress or A.N.Other blogging tool. They could have even used their own blog search engine...
It's interesting seeing what people are talking about as you browse news sites or popular blogs - however, with this tool you never forget that you're only hearing part of the conversation.
It's a pity.
This isn't the two way web we were promised - it's more like the one and a bit way web.
And the bit isn't very much at all.