Currently it can only be installed using
$ composer create-project "kylecannon/magento" -s dev
The package of corsecourse could be replaced with any other trustworthy Magento package of your choice.
Of course that's not satisfactory, since it can't simply be installed by adding it as a requirement in the composer.json.
The magento-composer-installer
The magento-composer-installer
project currently doesn't support installing Mage_Core_LatestMage_Core_Latest
due to missing dependencies. If nobody else fixes that in the http://packages.firegento.com build script, I want to do it at the next hackathon at the altestlatest.