From the Times "What's Online" column:

Naturally, readers are drawn to the blog, which picks up where the book leaves off. And unlike a lot of writers who blog their books with a seeming reluctance, the authors, Steven D. Levitt, an economist, and Stephen J. Dubner, a journalist, take to it with the same zeal they applied to their book, and the blog is abuzz with activity.

For example, Mr. Levitt tells of an e-mail message he recently received from a fellow trend-tracker, Malcolm Gladwell: A man approached Mr. Gladwell at the Toronto airport, asked for an autograph, and pulled out a copy of "Freakonomics" for him to sign. "We are totally co-branded!" Mr. Gladwell wrote.

They already were. Mr. Gladwell's name, affixed to his blurb ("Prepare to be dazzled") appears above Mr. Levitt's and Mr. Dubner's on the cover of "Freakonomics." And Mr. Levitt heaps praise on Mr. Gladwell several times on his blog.