According to the official copyright website (copyright.gov):
“The practice of sending a copy of your own work to
yourself is sometimes called a poor man’s copyright.
There is no provision in the copyright law regarding
any such type of protection, and it is not a substitute
for registration.”
Type "poorman's copyright" into Google and you'll find
several hundred articles written about this. Use it if
you must, but use it knowing the facts.
http://www.snopes.com/legal/postmark.asp
Poor-man’s copyright.
http://www.legalzoom.com/articles/article_content/article13787.html
Try this yourself:
Take a blank envelope and write your own address on it.
DO NOT SEAL THE ENVELOPE. Mail the envelope to
yourself. In May 2011, when a new big hit movie comes
out, pick up a copy of the script, scan it into your
screenwriting software, change some stuff around, print
it and put it into the envelope with the June 2009 date
and THEN seal it.
You now have an envelope posted in June 2009 with your
work version of the script that is AMAZINGLY similar to the
version in the theaters. Any well paid copyright lawyer can
poke holes in the “poorman’s copyright”. Is it really worth
saving $30?