I used to send an email (and still do sometimes) but sometimes it seems that there isn't a point. Usually I think I just give up hope and figure an email that they delete isn't going to make a difference in the end. When it's a job that I actually want (or with a company that can provide that job later) I go out of my way to make sure I send it and make it known I want to work there.

Occasionally I want to send the email but feel really awkward about asking for contact info in an interview (especially when there are more than one person at once). When they give a business card or I have their info beforehand I generally do even if it's just an awkward 'thanks for the time'. Somehow I feel like if they wanted me to contact them they would provide the info. I know I should make the effort to get the info but as you can see from my mindset above I generally feel like I bombed and it's pointless anyway.

Back on topic, I recall having to do interview stuff in classes (can't remember which...language arts maybe) in both high school and college and it was always brought up to send a thank you. Maybe people didn't pay much attention.