There is a vast difference between:

a) Keeping a personal archive for private use b) Reproducing small parts of collections or larger works for commentary, and c) Reproducing, publicly, a giant archive of copyrighted material.

What Dave is doing is roughly equivalent to recording a few days worth of radio and streaming it back over the internet to the public at large a few days later.

We can talk all day about what Dave might have done instead that wouldn’t have been blatant copyright infringement but, at the end of the day, he didn’t do any of those things.

Also, I believe this site has some examples where Dave took down a tweet or a blog post and either this blog or others have posted either a screen shot of them or the text verbatim. (And he’s definitely done the same thing). So technically in those cases you (and your sub-authors), as well as Dave (when he’s complaining about stuff) are engaging in copyright violations from that.

It’s definitely a grey area–especially if you let the world see your stuff.

When you post a tweet, you are giving Twitter a revocable license to publish and display your tweet… that license doesn’t extend to Dave Winer.

It’s actually quite significantly different from anything that could really be called “caching”, which tends to be ephemeral and private to some service, application, or individual. What this really is is “archiving” and it’s a clear violation of copyright.

Twitter is mostly people posting to the public. If there’s clear examples of him posting info that’s meant to be private (such as private tweets), you might have a point.