It's not a HOAX!!. Yahoo Messenger is truly under maintainance. Well, it's quite lucky since they are doing their maintainance on Saturday when most people don't work so those who are used to use YM! will not have too much problem on their work (besides, it will only be down for about two hours if there's no more delays). This means that Windows Live Messenger will also affected due to this maintainance, since they have made a cooperation between Microsoft and Yahoo to let their IM client chat to share their network services.
Please note that this maintainance only affect YM! services, not the other Yahoo! services, such as Yahoo Mail. Let's just wait and see for now.
I got this prompt when i tried to login to YM! just now: