I've heard that plenum cable is like putting a spunge in the ground. Can't verify that myself, as I've never done it, but that's what I've heard.
Avaya also has an outdoor Cat5e and a Cat6. I don't think I would call it a direct buried cable though. In conduit is the only place I'd use it anyway. Had some 5e lying around here not to long ago. I think it was either Comscope or General Cable, can't remember. But again, not direct buried. That I think would be diffficult to find.
I'll put in a second for the lightning protection. Even here, where lightning is very minimal, and lightning strikes even less likely, every copper cable that leaves a building, has protection on the both ends.
Justin T. Clausen
Physical Layer Implementation
California State University, Monterey Bay