I have helped with the hiring at my company and I can't say whether this is common or not, but often at the end of a long day of interviews we meet to discuss candidates and forget who said / did what. Yes, we should have taken better notes. But we did not.
So while I don't think a simple thank you note would do much, I do think a thank you note with reminders of things we talked about in the interview that you bring to the table can definitely be a plus. It doesn't have much to do with thanking anyone (although that is nice) but it helps refresh our memories and can make sure your positive traits are not lost in the often chaotic messes that are interview days. And unlike your cover letter / resume, you can now address specific needs of our company that came up in the interview. Example: "During the interview you spoke about the need for programmers to understand networking as well, and I agree. My certification in blah blah blah network technology, combined with my years of programming experience in blah blah blah, make me an excellent candidate to fill this role, blah blah blah."
Another thing we talk about in the end of interview day meetings is candidate enthusiasm for the position. The thing is, it is often hard to gauge this in an interview. So receiving an email afterwards that restates your interest in the position after you have came and seen the working environment, met with managers, etc. helps us determine that you are still seriously interested.
So I definitely vote yes on thank you notes, just make sure you use them to show enthusiasm for the position and refresh us on what you have to offer.