I dunno, the brilliance of the line goes far beyond the line itself. It's obvious to most anybody that if you do what glados says, you probably won't be getting any cake - afterall, she does try to kill you just before offering you cake. What makes it great (for me at any rate) is given the fact that her plot is unraveling, and in spite of untold amounts of knowledge about human psychology and whatnot, the best she cam come up with when pressed in order to motivate you is "i'll give you cake". It's the sort of peace offering a child would offer - implying a naivety behind the muderous mischef that she consistantly expresses.