It's not when you are "one of two finalists"; the employer could decide that you are not the newest American Idol.
It's not even when you get the handshake from the final person on the interview conga line. The corporation hires you, not the person doing the interviewing.
It's not even when you have an email saying "we love you! come work for us!"
If you don't have a letter (or better yet, a contract) that you can touch and lock up in your safe deposit office, you may not have the job!
Follow through, if needed; ask politely for a letter specifying your salary, title, terms of employment. Here are some ways to follow up, either for an interview from which you've never heard back, or for a clearly stated job offer: