Leaving a small child in a hot car for a long period of time can be dangerous to his or her health, and possibly even fatal. The Kalamazoo Gazettehas some details on this topic.
This has been an area of growing concern for parents and child safety advocates. It is important not to become overly paranoid about it, as 340 children have died in this manner in the last decade--a disturbing number to be sure, but hardly a cause for the sort of hysteria that safety issues sometimes generate.
Yet awareness about the issue is nevertheless important. Parents should not leave their children locked in a car unless some emergency absolutely requires it. This is especially true in the hot summer months.
The Chambersburg Public Opinion has more details on these dangers, including statistics and suggestions. For instance, remember that children's bodies heat up three to five times faster than adults', that cars heat up much faster than you might think (even with the windows down) and that it is important to take a few seconds to check the back of the car before walking away from it. The Houston Chronicle has a good description of how such tragedies can happen even to children with loving, caring parents--one moment of forgetfulness is all that is required.