I'm not sure how a "factoid" can be prefixed with "to the best of my knowledge," but here is some cold hard fact for you:
A work can enter the public domain via expiration or forfeiture.
EDIT: A bit of clarification. Some people get confused because copyright actually affords you two different sets of rights: Economic rights, which everyone is pretty much on the same page about (once your work is in the public domain you can't charge for it later). You also get moral rights (right to attribution, right to maintain the integrity of you work, etc). These rights vary wildly by jurisdiction, but are rarely the ones that get argued when a work is in the public domain.
Graber, H. B., & Nenova, M. B. (2010).
Intellectual property and traditional
cultural expressions in a digital
environment. (p. 173). Edward Elgar
Publishing.
A work can enter the public domain via expiration or forfeiture.
EDIT: A bit of clarification. Some people get confused because copyright actually affords you two different sets of rights: Economic rights, which everyone is pretty much on the same page about (once your work is in the public domain you can't charge for it later). You also get moral rights (right to attribution, right to maintain the integrity of you work, etc). These rights vary wildly by jurisdiction, but are rarely the ones that get argued when a work is in the public domain.