IMHO. If there were no problems with Job's health he would do the responsible CEO thing and do an interview with a tech site. But he hasn't. Either he doesn't care that Apple stock is going down because of this news or he can't.
Why would it be responsible for him to do an interview with a tech site. Steve Jobs is a little bit bigger than that. If he wanted to an interview he would do it on prime time TV (Larry King or something).
If Steve was sick it's debatable whether or not he would have to tell the board. But if he's healthy, he doesn't have to say anything.