I needed to move all wiki pages between two repos.
git clone git://github.com/darwin/move-wiki.git
cd move-wiki
git submodule init
git submodule update
Tell me who you are. Set your GitHub credentials into global git config (see account page):
git config --global github.user darwin
git config --global github.token fffffffffffffffffffffffffffc122c
rake pull_wiki user=defunkt repo=grit dest=some/dir
rake push_wiki user=mojombo repo=grit source=some/dir