Notch has posted on twitter about the subject of Herobrine.
@xnotch Is "he" a real entity ingame or is it just a successful_troll.swf? (Posted by RavenBurga)
@RavenBurga It's not real. =) But it might be soon!
Although this confirms that there is no Herobrine, it suggests that Notch plans to add him as an entity in the near future, presumably acting in a similar way to the faked Herobrine.
At Minecraftcon 2010, Notch confirmed Herobrine will be in the game, probably in a far future seecret friday update.
While many groups disagree over Herobrine's existence, the majority opinion is, until the friday update he is included in, he does not exist. Although Herobrine may very well appear in Minecraft due to the prank, and likely when is least expected.