Romeo, as I understand it,is the development branch of Mint. Upgrades and improvements for the next release are available from the Romeo repository if you wish to test them. For example, today's mintBlog has announced a new version of mintUpdate which can be found in the Romeo repository. This is the link:http://packages.linuxmint.com/pool/romeo/m/mintinstall/mintinstall_4.0_all.deb
I think the name Romeo is a play on words. All the Mint releases have been female names, Romeo being male suggests that it is not the 'norm'.
I can understand that throwing the name Romeo around is a bit confusing for new people to Mint. Perhaps there should be a sticky somewhere that explain it better than me from someone who really knows.