Just curious is it crucial?
It's one of the reasons I like academia that I don't need to care too much the boring stuff like wearing suits.
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Standard advice for any job interview, academic or not:
Unless you think quirky would definitely be to your advantage, dress fairly conservatively. It's easier for interviewers to think of you in terms of how you differ from a standard quantity, and you want them to focus on the differences that are in your favor. It may also be taken as an indication of serious interest, and of respect.
Of course, on a college campus, "conservatively" covers a broad range from business casual (with or without sport coat) to t-shirt and a clean pair of jeans (but avoid the shirts with political messages unless you know the interviewer's biases).
So the real answer is: If in doubt, call the department's office (or the professor's administrative assistant, if he has one) and ask them what's customary and appropriate. They'll know. Then consider going toward, or a half-step past, the well-groomed end of that scale.
Personally, I can't think of a single 'genuine' post-doc I've known over the years who had enough mental capacity left to even begin worrying about what to wear.
On a more serious note though, based on my experience within the field of computer science in The Netherlands (seeing as regional differences do matter), taking some care not to pick an indecent or dirty pair of clothes should be the minimum. Suits would be way overdressed.
In a postdoc interview, you are presenting yourself as a professional, following the standards and conventions of your field. What exactly is that level of dress depends somewhat on country and on field.
A good metric to gauge yourself by, however, would be what the postdocs and pre-tenure faculty tend to wear at the conferences in your field. If you dress following the mode of that population, then you are sure to not have clothing be an issue.
Dressing in a fashion that expresses awareness of the context would be ... oh-so-wise. :) Whatever the local standards are, contravening them is not to be lightly undertaken. :) E.g., to communicate that you are an iconoclastic prophet, breaking all the iconic dress-codes is de rigeur, but if you want to convince people that you can help them in their previously-understood enterprises, ... try to show them that you "can be/are like them"... :)
If you feel the job is already yours, then it doesn't matter what you wear.
Otherwise, dress in a way that expresses your enthusiasm for the job -- including the ability to dress however you like once you're hired.
You needn't wear a suit, and you don't need a tie. But you need to wear (I'm assuming the masculine gender here) nice slacks and a tailored shirt (i.e. with buttons). No sandals or flip flops, but if you're not comfortable in dress shoes, then a pair of sports shoes that are fairly new looking. The shirt should not be loud or Hawaiian. Make sure the shirt and slacks don't clash.
There. Was that so hard?