Flat panel collectors weigh about 200-250 lbs. Thermosyphon systems add the weight of the water storage tank. Residential scale evacuated tube arrays weigh about 175 lbs.
More serious considerations, from a structural perspective, are wind-induced loads and concentrated loads. Remember that the snow or ice that used to be spread out evenly over your roof will now be concentrated at the mounting feet. Most of us know what high heels do to a wood floor.
For these reasons, a structural analysis is often required by the building officials who review plans and issue permits for solar systems.
It is a good idea (and required by some incentive providers like Energy Trust) to install solar panels over a roof which has 15 of more years of life in it. Some folks do a partial re-roof under the area of the solar array even if they are not prepared to do a complete roof replacement.
Fire Marshalls are now asserting their authority in the permit/plan review process to make sure they have room to work in the event of a fire in the home/building. A qualified solar contractor will help you work through all of these issues; including working with a structural engineer if that becomes necessary.
With an increasing body of information available, the added assurance of a successful installation that these steps provide adds some expense but not a deal-breaker to your solar system.