Couldn\'t find anything in the github documentation and also here on SO. But I was wondering if there could be a http://foo.github.com
for a private repository
According to GitHub Pages documentation:
All project repositories are ready to use the generator for publishing. However, please note that private repositories will publish pages that are public.
So no, at this time there is no way to create private GitHub pages from a private GitHub repository.
EDIT:
A workaround for some situations that might be helpful is to simply rename the repo to something other than the GitHub pages format while you want it to be private (for example in a development phase) and when ready to make it public then correct the name. Obviously this still doesn't help if you are looking for a way to publish pages that have authentication, but if you just want to hide a GH pages project while it's in progress, this could help.
Alternatively, there is a project called Jekyll Auth that GitHubber @benbalter made for such use. Jekyll Auth provides a basic authentication wrapper for jekyll projects, including GitHub pages. See the repo's README for use.