[Lawtan]

Hmm...
Well, I'd say it would depend in part on the intent of the art, the intent of the tracing, etc.

Examples I would not consider theft:
1) Sketches trying to learn from "the masters" (Leonardo, Salvador Dali, etc.)
2) Art that is meant for tracing/practice drawing. (How-to's)
3) Using multiple drawings as research/reference for your drawing.

What is theft.
1) Using references that request citation without citing.
2) Tracing/copying another's art without citation/consent.

And...that's about it for me.