Documentary theatre is theatre that wholly or in part uses pre-existing documentary material (such as newspapers, government reports, interviews, etc.) as source material for the script, ideally without altering its wording.
Guardian Unlimited Excellent article discussing the reemergence of theatre of fact dramas in contemporary UK theatre and what effect this is having on the fictional world of the modern playwright.
McGraw-Hill Useful definition of documentary drama.
Robert Hubbard Very good description of what documentary drama entails (scroll down).