I already have a page where significant announcements show up at a wide variety of frequencies. It's called "webmail". Or an activity feed.
The point of a dashboard is that no navigation is required beyond turning your head and glancing. Dashboards are for things that you want to see by accident, or while waiting for a phone call. A widget that usually isn't there when you turn your head might as well not be on the dashboard at all. If you have to click to see it, it can be in a separate app or site. If you have to wait for a specific date and time to see it, it might just as well arrive in your mail, Twitter stream, or SMS.
The main difference in my eyes is that activity feeds and messages can create a backlog. The user experience isn't quite right either--different types of messages still look exactly the same. But different dashboard widgets could have radically different user experiences.
The point of a dashboard is that no navigation is required beyond turning your head and glancing. Dashboards are for things that you want to see by accident, or while waiting for a phone call. A widget that usually isn't there when you turn your head might as well not be on the dashboard at all. If you have to click to see it, it can be in a separate app or site. If you have to wait for a specific date and time to see it, it might just as well arrive in your mail, Twitter stream, or SMS.