In the August 4, 2005 column of "Dear Prudence" (available at http://slate.msn.com/), Prudence rails about the use of "No problem" instead of "You're welcome" as a response to "Thank you." She seems to be most annoyed when "No problem" comes from someone who is being paid to serve you, such as a waiter or a sales clerk. Since they are being paid, it shouldn't matter whether anything they were being asked to do was a problem. I disagree with Prudence. I think that "No problem" is just as valid an answer to "Thank you" as "You're welcome."
First of all, I consciously began to use "No problem" instead of "You're welcome." I started doing this while I worked as a student assistant in an office at a university. As part of my job, I was constantly helping people with their computer problems over the telephone. When the problem had been solved, the person would usually say "Thank you." I started out saying "You're welcome" in response. However, this felt awkward to me. It seemed as though I was taking the credit personally for doing something that was part of my job. So I began to respond with "No problem" - as in "It's not a problem; it's part of my job." I don't think that, in the context of doing something that is part of your job, it is rude or inappropriate to respond with "No problem."
I would also like to point out that "No problem" is much like the Spanish statement "de nada." While "de nada" is usually translated as "You're welcome," it's literal translation is "It is nothing." How is saying "It's not a problem" different from saying "It's nothing"? If it's good enough for Spanish, it's good enough for me.
I take comfort in the thought that Prudence probably won't be able to stop people from using "No problem" just by complaining about it. If people who dislike the phrase "No problem" are seriously concerned about rudeness, then they shouldn't respond by saying "I don't care if it was a problem for you!"
(By the way, I really enjoy reading "Dear Prudence." That doesn't mean I can't disagree with her every now and then!)