Anyone else notice that when GLaDOS was put into the potato at first, she said she couldn't lie; that she didn't have enough power from the potato. Then she tells you when you stab her into your portal gun, she will get 1.7v, and be able to lie again?
But, seemingly, she never lies for the rest of the game. VALVe wouldn't just have her say that. That has to relate to the story somehow.