In the issue of the Notices of the American Mathematical Society, which arrived in the mail today, I learned that Structure and Randomness: Pages from Year One of a Mathematical Blog will be coming out soon.
I saw a draft copy of it when Tao posted it on his blog back in April; he announced the book was going to exist here and posted the draft here. The book is a collection of Tao's posts on various open problems, expository posts, summaries of lectures, and so on, and is in large part intended to be, so far as I can tell, what one might think of as an "archival" version of the blog. A lot of Tao's blog posts tend to be longer than typical blog posts, so his blog seems particularly suited for the book medium.
The title of the draft version is "What’s new - 2007: Open questions, expository articles, and lecture series from a mathematical blog", which describes the content well -- but I like "Structure and Randomness" better. Here's a long post I made back in July of 2007 on "psuedorandomness" in the primes, in large part inspired by watching a talk Tao gave entitled "Structure and Randomness in the Prime Numbers".