The way Octopress deploys to Github Pages is a little funky. When you setup your blog for deployment to Github Pages, a second copy of the repo is instantiated in ./_deploy, and is checked out to the master branch.

You create and edit posts in the source branch, in your project root. During deployment, your site gets generated, and the updates copied to _deploy. From there, a commit is made to master, and a git push is performed, updating the website.

When you clone your blog to a different computer, you need to make sure it is setup in the same way, like so:

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# Assuming that you have your Ruby environment configured
$ git clone -b source git@github.com:username/username.github.io.git username.github.io
$ cd username.github.io
$ git clone -b master git@github.com:username/username.github.io.git _deploy
$ bundle install

Thank you to zerosharp.com and dblock.org for helping me figure that one out.