Those are all rumors. The first Herobrine encounter was a hoax arranged by some guys on a server. There's never been a code to summon him, nor a texture file in the game for him. He's about as real as Captain Picard (can't say Bigfoot because there is actually a legitimate chance.)
By the way, if you see any videos on YouTube of SMP Herobrine encounters with summoning and so on, those are mods. Notch had hinted before of adding Herobrine in the game, but once he was confronted with the rumors he said that Herobrine has never been nor ever will be in Minecraft, aside from mods of course, but a similar super rare but powerful mob may be added at some point.