Solar Panels typically weigh 3-4 lbs/sq ft. The dead load is generally not a problem for roofs, which meet current building codes. Older homes may require an engineering analysis.
More serious considerations, from a structural perspective, are wind-induced loads and concentrated loads. Snow or ice that used to be spread out evenly over your roof will now be concentrated at the mounting feet. For these reasons, a structural analysis is often required by building officials who review plans and issue permits for solar systems.
It is a good idea (and required by some incentive providers such as the Energy Trust) to install solar panels over a roof which has at least 10 more years of life. Sometimes you can do a partial re-roof under the area of the solar array if you are not prepared to do a complete roof replacement.
Fire Marshals 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 or building. A qualified solar contractor will help you work through all of these issues, including working with a structural engineer if that becomes necessary.