Question: For the 4 jobs that I have worked at, I dressed formal twice, business casual once and more casual once. How important is the dressing code to programming interviews these days? Will the employer or interviewer not take you seriously if you come in casually: plain shirt and khaki pants with casual walking shoes(non-sneakers)?

Answer: 
If you already know what the dress code of the company is, dress consistently. It doesn't hurt to be *slightly* smarter.

Otherwise, the employer will take you more seriously if you phone and ask before you turn up.

If you don't ask and you turn up looking smarter than the dress code, I don't think anyone will think less of you. If you turn up looking less smart, they will.

(If this sounds hard, imagine what it's like to be female and often get the response, "Shirt, no tie.")
Unless you show up wearing a white tuxedo, you will almost never have points taken off for being too formal, so why chance it. I would say suit by default, and button up shirt and khakis at an absolute minimum and only if you happen to know that the place is super casual. Always try to out-dress the person interviewing you.