I know that announcement will be quite important for the winning bid team - but it's just a matter of course to manage that. It would be good for them to have some notice, perhaps a day or two, to prep a basic news release highlighting the news.
I don't think there's a huge risk of the news getting out. Media would really need direction from the winning committee to get the full story. So generally it would be better to be ready and confident with the news/announcement.
I don't believe WMF formally announced the winner of the upcoming year's WMania last year, but it might be a good way for us to help draw attention globally to Wikimania in more general terms... perhaps in the same big press release we could announce the 2010 location and then hopefully we'll have media registration for Buenos Aires ready? Or if we haven't got anything new to report for BA, then we could just have a few quotes from both teams talking about Wikimania.
Thanks!