AdWords doesn't bill you until the end of the month or until you reach a threshold before then. See "[SIZE=-1]The AdWords Billing Cycle" at[/SIZE] Google AdWords: Learning Center for details.
I don't remember how MSN adCenter works. It might be the same way, though.
Edit: Just checked... adCenter is similar. You run a tab and it goes to your credit card when you read $50.
