Interesting.
I'm recently out of graduate school and I've been interviewing for full-time positions recently. I send a thank you email 8-24 hours after every interview, with very few exceptions.
In contrast, I've been surprised/disappointed by the general vibe I've gotten in the interview process with a number of employers, large and small. Put most simply, there's little respect shown for candidates' time or situation. (This has been echoed through watching other classmates' experiences as well, so it's not just me being a low-value candidate)
The specifics vary, but it always clearly a one sided interaction and little effort is put in to relations with potential employees except for the most high-profile, rock star candidates.
To me, that seems to be a serious failing - I appreciate the mindset that says you should treat employees with the same mindset that guides your interactions with customers - especially in high-talent or high customer contact industries where employee satisfaction translates quickly into real business value.
So, I guess my point is that I feel the job search process reveals more about society in general when viewed from both sides.
Still, I will always send the thank you notes. I figure the place I want to work is one that appreciates the small gestures as well as the big ones...