Hemp is a form of cannabis, IIRC, but does not contain enough THC to get you high.
Found this:
Hemp Facts
"Industrial hemp has a THC content of between 0.05 and 1%. Marijuana has a THC content of 3% to 20%. To receive a standard psychoactive dose would require a person to power-smoke 10-12 hemp cigarettes over an extremely short period of time. The large volume and high temperature of vapor, gas and smoke would be almost impossible for a person to withstand."
The only problem is the appearance, I believe--they look very similar. But industrial hemp could be genetically modified to look different.