Read Desireé Rumbaugh's response:
Weight lifters can make good yoga students, and they will benefit greatly from a good education in proper bio mechanical alignment principles. Without full knowledge of proper alignment, however, it's probably not wise for weight lifters to try to do yoga, as their limitations will frustrate them at every turn.
Weight lifting causes the outer body to become harder. The muscles that are built are shorter and thus more powerful. Yoga causes the muscles to hug to the bones, become longer and leaner, and, ultimately, become more functional in range of movement. Weight lifters can lift more than their body weight; yogis are skillful at lifting and supporting their own weight in all positions, including up in the air.
At some point, these yogi/weight lifters will probably have to make a choice, because there's not enough time in the day to be serious about both practices. But even if they choose weight lifting, the principles of stretching and breathing that yoga offers will still be very beneficial to them.
One key piece of advice I'd give is that you remain cheerful and patient with these students. If they're coming to yoga, it must be because there's something that attracts them. Weight lifting is mostly an outer-body strengthening, while yoga is a journey inward to the soul. Each of us must choose our own path in this lifetime, and the journey cannot be rushed. I know some former yogis who have chosen to lift weights and meditate rather than practice hatha yoga, and they are very happy with their choice.