You only have to be online once when you install the game.
This was flat out stated initially on game release, and turned out not to be true do to a bug, which Valve acknowledged, which I myself experienced, and said bug has been sqashed.
After about a month, I was forced to reconnect and update my steam and half life 2 etc.
Instead of flat out stating this is true, let's just say as far as we currently have knowledge of, barring unforseen circumstances, you most likely only need to connect to the internet once during installation and setup.