"I was planning a nigh-complete rewrite for the book - less than 50% previously published sentences, say. Would it be a problem if the ideas, but not sentences, all exist elsewhere?"
You might need to remove some posts from the net if you decide to use them as is in the book, but if it is all modified, there shouldn't be a problem AFAIK.
Ideas can't be copyrighted, and you wouldn't be the first person to turn blog material into dead tree.
One million cumulative daily visits! Woot n' stuff. Also we're in the top 5,000 of all blogs on Technorati, and one of the top 10 econblogs by Technorati rank.
Seems like a good time to mention that I'll be appearing at Penguicon, a combination open-source/science-fiction convention in Troy, MI, Apr 18-20, as a Nifty. I'll be doing an intro to Bayesian reasoning that you probably don't need if you're reading this, possibly a panel on the Virtues of a Rationalist, some stuff on human intelligence upgrades, and definitely "The Ethics of Ending the World" with Aaron Diaz (Dresden Codak).
After the jump, you can see some proposed cover art for the blook.
For the benefit of the humor impaired: Yes, this is a joke. Erin, my girlfriend, Photoshopped this when she heard I was planning to do a book.
This is all taking longer than I expected - as expected - but I do think I'm getting there.
My current serious strategy for the blook is as follows:
If you've got more experience in the publishing industry and you see some reason that any of this won't work, i.e., "No one will talk to you if you've ever done anything with Wowio or Lulu" or "Today's readers don't want short popular books, they want 500-page tomes" or something like that, please email me or comment.