There is a common belief that if someone infringes a copyright, and the
copyright owner does not sue or otherwise put a stop to the infringement,
the copyright is lost and the work goes into the public domain. There is
some pre-1988 law on this (e.g., Stuff v. E.C. Publications, 432 F.2d 143
(2d Cir., 1965) and Transgo v. Ajac Transmission Parts, 768 F.2d 1001
(9th Cir. 1985)), but it seems to derive mostly from the fact that the
copyright holder had acquiesced in the publication of the work without
notice back when notice was a requirement. It was the publication
without notice, and not the lack of enforcement, that actually worked to
put the work in the public domain. This is forfeiture of copyright, not
abandonment. Because the notice requirement is now gone from copyright
law, these cases don't have much weight today.
I can't find anything that supports the idea that failure to assert a
copyright against an infringer can alone lead to placing the work in the
public domain (if you have any authoritative information on this, please
drop me a note at one of the addresses listed in the introduction). Of
course, circumstances may be such that the ability to sue a particular
infringer might be waived (e.g., a statute of limitations may expire (see
section 3.4), or if the infringer has reasonably relied to his or her
detriment on the copyright holder's failure to sue, the doctrine of
laches may bar a suit), but that's only with respect to that particular
infringer, and does not affect the status of the copyright with respect
to others.
http://www.faqs.org/faqs/law/copyright/faq/part2/