Yahoo Mail Security Risk.
It may be a temporary glitch, but I have a hunch that the reason Yahoo Mail users are suddenly realising that their signout icon has vanished is because Yahoo are trying to increase their users' session time. With no annoncement, it now appears that you have to locate your Yahoo home page and find a signout icon there (amid the other Yahoo services they'd like you to use).
Already I see confused and worried people on the help forums, but what about those who don't realise and leave their webmail accounts wide open? As I say, this may be a glitch, but if it's an attempt to lock-in users, it's a very bad example of maniplulting the user experience for the sake of the business model.
UPDATE: It appears to have disappeared - was it a glitch or a sign of things to come?