In an effort to bridge the narrative gap between Portal 1 and Portal 2, it seems like Valve are going the only route they think fits: comics.
IGN have a first look
at the books, but the Halflife2.net
forums mention that we're likely to be introduced to a new character who was hinted at through Portal 1, and speculation points at Doug Rattmann; the purported graffiti artist who first told us all "the cake is a lie". Of course that might not be all they touch on, as it's long been rumoured the Half-Life universe and the Portal universe are one and the same, so maybe this internally produced material will finally shed some light on that.