Shadow has always been the "off-spec" for priests, from Classic, where it was questionable whether Shadow was viable for anything but leveling and PvP, to BC, where Shadow provides superb utility but limited DPS. Now I'm not saying Shadow will suddenly become the spec of choice for raiding priests, but the game is changing significantly in patch 3.0.2 and Wrath of the Lich King. Our coveted mana return is getting nerfed and spread around to Survival hunters and Retribution paladins, as well as us. But to compensate, our DPS is (supposedly) being brought up to par with the best of them.
A lot of this is coming in the form of a new and improved Mind Flay. Previously the "filler" spell that you cast during your rotation when you couldn't cast anything else, its spellpower coefficient is now being bumped up greatly, and perhaps more importantly, it can now crit! No longer must you shun crit rating to be a decent shadow priest.
Of course, there are also some dramatic talent tree changes - almost every talent is modified in some way. Let's start from the top and work our way down.