It's got to be heavy music in my book. It's kind of the natural progression of metal vocals. There is some stuff that has only half screamo, like maybe early Mastodon.
It's hard to appreciate this kind of stuff, but Naplam Death was an originator of this I call Difficult Music. See example below.
Lefty: I've never heard screamo with non-numetal riffs. Turn me on to some if you have any examples.