yaaa i thought the same it might b a tatoo if there is any 3rd eye we may b knowing it in the battle b/w itachi n sasuke !!!I don't see a third eye Oo
*sees it now*
Looks like either a tatoo/drawing on the forehead, and is probably for a minor jutsu. We know exactly why Itachi poked Sasuke all those times, and it was to transfer basic instincts on how to perform certain techniques.