I have noticed the tendency the past few years, for someone to respond “Not a problem” instead of “You’re welcome”, when someone says “Thank you” or “I appreciate the help” to them.
Boss: "I need you to have your re-writes done by Tuesday."
Employee: "Not a problem."
While the employee might be accurate, that performing their job duty is not an undue stress on them, the boss typically has that consideration low on her list. She simply wants her document completed and turned in so she can move it forward. The employee is not doing the boss a favor. He is doing his job.
[Author's note: This posting was originally posted back in the summer of 2007, but I edited it, in 2013, to separate the two sections for readability.]