This is critical, asking for you interviewers business card is a sign that you are planning on following up or that you are very serious about the opportunity. A professional interviewer should offer it to you but if not please remember to ask. This is essential for you to be able to send a thank you email or card for their time and consideration.
The Fortune is in the follow up and by forune I mean job. Treat the interview like a sales meeting and close the deal.