The Mighty Jerd writes an article examining history of how comic books have dealt with the deaths of characters.
In the beginning comics were a light-hearted affair and the only people who died in comics were supporting characters in origin stories or in flashbacks (very often off-panel); people like Uncle Ben or Thomas and Martha Wayne. As comics, and their audiences, matured occasionally an ongoing auxiliary character would meet their demise; these were the Gwen Stacys and Iris Allens of the comic-verse. These deaths were very rare, almost always came as a huge shock to the reading audience and were used in a way to drive plot or to provide new motivation for the main characters.
The Wikipedia entry on comic book deaths is also a pretty decent source of info.