Yes, it's true, says a study in the Proceedings of the National Academy of Sciences (via the New York Times): sildenafil, otherwise known as Viagra, is a bona fide cure for jet lag.
According to a recent article in the New York Times, a 2011 survey by Pfizer found that 80% of pills sold online as "Viagra" were counterfeit drugs that only contained about 30% to 50% of the active ingredient sildenafil.