The moment in which you really start feeling that you are actually leaving is when you have to return your assets (e.g. your laptops, and a few other small things). You give back the Macbook that you’ve worked on for two years, and that’s when it hits you. “Yes, I am leaving. It is real”.
My friend Simone just left VMWare to start a new professional path. He published an interesting article on the mixed feelings of leaving a job.