As announced earlier, the Communications committee is coordinating the use of the site-wide notices on Wikimedia Foundation sites. Right now, we need to call attention to Wikimania and encourage people to register, so starting sometime tomorrow we will be putting up a brief project-wide notice about this. We expect the notice to run for about a week.
When this use of the site notice ends it should be blanked again (except for any separate notices to anonymous users being used for fundraising). We don't want these to be overused, so an extended silent period for the site notice should follow. After that, the next use will probably be for the fundraiser, assuming that committee is organized and a date settled on.
--Michael Snow