Lying is a form of manipulation, and humans don't want/like to be manipulated. If the CEV works, then it will understand human concepts like "trust" and "lying" and hopefully avoid it. The only situations where it will intentionally manipulate people is when it is trying to do what is best for humanity. In these cases, you don't have to worry because the CEV is smarter then you, but is still trying to do the "right thing" that you would do if you knew everything it knew.