From Urbanlegends.about.com

"It's odd, to say the least, to encounter a passage attributed to Julius Caesar which, as far as I can tell, never appeared in print before 2001. It's equally odd that while the passage is cited in dozens of Internet discussions concerning post-9/11 political developments, it never turns up in articles on Julius Caesar himself. If it is to be found among his own writings, no one has yet been able to pinpoint where.

It has also been attributed (most famously by a red-faced Barbra Streisand) to William Shakespeare, who presumably would have composed the lines for his historical play, "Julius Caesar" — but they're nowhere to be found there, either. Apart from one small phrase, "And I am Caesar," which vaguely echoes the closing words of a couplet in the play ("I rather tell thee what is to be fear'd / Than what I fear; for always I am Caesar."), the language is anachronistic and distinctly un-Shakespearean. The words "patriotism" and "citizenry" were unknown in Elizabethan England. The Bard's Julius Caesar spoke in iambic pentameter, not mediocre prose.

Short of the culprit stepping forward, there's little likelihood of ascertaining who did write this politically convenient baloney, but we know it wasn't Shakespeare and we can be reasonably sure it wasn't Julius Caesar. It does, however, bear all the markings of a "classic" Internet hoax."

http://urbanlegends.about.com/library/bl-caesar-quote.htm