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: 
**Short answer: Unless stated explicitly otherwise, always formal.** 

It is not about being taken seriously, it is about *interviewee* showing respect to the *interviewers* and taking the job application process seriously.
A suit is generally considered to be the de facto standard business attire, meaning that when going someplace to conduct business, it is always considered appropriate to be worn if you are not already familiar with who you are meeting with. Likewise, on an international basis, nobody should be offended if you show up to an important meeting wearing a suit and in some cases, people will be offended if you do not wear a suit. 

An interview is a formal business occasion and as such, a suit would be considered appropriate for the occasion unless you are explicitly told by the folks you are interviewing with that they have a "casual office" or explicitly tell you not to wear a suit. 

As a matter of personal practice I generally wear a suit when interviewing and when I was told not too I still wore business casual. However, when I have had to interview candidates I've also extended the same expectations and wore a suit for the occasion myself.