Sony were attacked (for what it feels like) months ago now, and even up to today, the PlayStation Network (PSN) is still down. Sony are continuing efforts to confirm the security of the network infrastructure as well as working with other companies to confirm the security of the PSN. Sony have said through their PlayStation Blog that they've begun the process of restoration. Also saying "additional comprehensive system checks and testing are still required, and we must complete that process before bringing the systems online."
Sony refuse to enable the PSN until they are satisfied with the security in the now-updated networks. PSN users just have to hold their breath a little bit longer.