Khoi Vinh takes a minute on his site Subtraction to discuss his policies on taking influence vs. inspiration (remember that conversation?):

The problem is that there’s such a blurry line between plagiarism and emulation that it becomes difficult to make unequivocal judgments on any given design that might bear some resemblance to mine. […] That’s what’s most important to me: that the design not simply be an instance of paraphrasing my design ideas, but that it’s a product of your own ideas. That’s how we all learn.

Give the whole article a read for some great perspective on where to draw the line during your creative process.