He changed his git config so that user.email was [email protected] (The email DHH lists on GitHub). GitHub then automatically says "oh, this is by [email protected], so let's put his avatar there and link to his profile as the author."

Changing the config is as simple as

    git config user.email "[email protected]"

And you can add a --global flag if you want that to be the default for all repos instead of just the one you're on.

Edit: There's also some way to do it with a --author= flag when you commit.