Even if it was an interview for a junior level (e.g. customer care executive) at a BPO, the attire of jeans and t-shirt / casual shirt is absolutely negative. It would reflect candidate's callousness and short term attitude. To appear smart, and to maintain a decorum of personal behavior is not at all like bending backwards to get the job. Yes, it is true that your technical skills are important to get the jo, but to succeed at workplace for a career, one needs to have strong values and graceful behavior, mutual respect and people skills as well.
The inputs provided by Mohit are quite fair. There is nothing colonial about them - I would say it is more in the mindset of people who see his comments that way.
I had also given my inputs for GD / PI., esp for B-School interviews. You may refer to that at http://www.rediff.com/getahead/2007/jan/22msg.htm.
