Hopefully I don't derail Lee's intended flow, as I'm going to touch only on magazines and newspapers, based on my experiences.
In writing articles for a magazine, not cartoons, they owned the property for two years. If I sought during that period of time to have my work published elsewhere, they had to OK it, take a cut, and get credit in the other publication where it appears.
Since this was a magazine with other media ties, they were also free to reproduce it on other platforms, such as radio, without paying me any extra.
But, after the two years had passed, ownership reverted back to me, and it was free game to have the article published wherever I wanted, however I wanted.
From my conversations with another magazine, it seems their contract is pretty much the same. But perhaps those arrangements are unique rather than the norm -- and don't even pertain to cartoons.
I also work for a newspaper, and we own the copyright to all original content (not wire copy or syndicated cartoons) that we publish. However, there is one exception -- the works of our local editorial cartoonist.
In her contract, we are purchasing the rights to first publication, but she retains the copyright. Therefore, the very next day she could have it published in our rival paper with no repercussions; other than her work may not appear in our publication ever again.
My two cents...or perhaps just one.