As I said in response to the feature requestresponse to the feature request
In support of being able to dismiss notifications, and in response to Diago's comment above, :
Are you ready to handle all the questions on Meta once they dismissed it? Wait there was this banner on the site I dismissed about x, how can I read it again*/*I never saw that, what is this new feature.
A "Notifications" page where these announcements permanently reside would nicely tie this up. I think that it would fit in well near the "about" or "faq" links. This way the notifications could be dismissed, but could be referenced by anyone who would care to look them back up.