Nope, polygraphs cannot tell whether you're lying. 1, 2
The polygraphs try to measure nervousness by registering sweating, breathing, blood pressure/pulse, and (I think) light trembling. They always calibrate the polygraph for each individual by asking questions the subject has no reason to lie about, to generate what is called a honest baseline.
The concept has some damning flaws, as no such physiological traits have been shown to consistently correlate with lying. Not all people get nervous about lying! And quite a lot of people would be nervous in an interrogation even when they were telling the truth.
There are actually a few signs that have been seen to correlate with lying, such as going at great lengths to avoid self reference ("then this happened", rather than "then I did this"), but nothing of the kind that could be picked up by a polygraph.
Richard Wiseman has this to say on lie detection through body language (from the book 59 Seconds):
For successful lie detection, jettison the behavioural myths surrounding the Anxiety Hypothesis, and look for signs more commonly associated with having to think hard. Forget the idea that liars have sweaty palms, fidget and avoid eye contact. Instead, look for a person suddenly becoming more static and cutting down on their gestures. Also, learn to listen. Be on guard for a sudden decrease in detail, an increase in pauses and hesitations and a sudden avoidance of the words 'me', 'mine' and 'I', but an increase in 'her' and 'him'. If someone suddenly becomes very evasive, press for a straight answer.