My guess is that the announcement will be at a similar time to the announcement of Mountain Lion last year, and the first developer release of Lion the year before that - around mid-to-late-February.
Now, last year they had those mini-events for journalists the week before. I'm wondering if this year they may do a small event (i.e. at the Town Hall) to announce 10.9 publicly instead? The slate for events seems like it will be quite empty until around WWDC, so they won't be worried about holding too many (last year they had held the education event in Jan and had the iPad event scheduled for March, so that explains why they went for the low-key hotel suite one-on-one meetings for ML).
I guess a lot of it depends on how much there is to talk about. Maybe, if the new organisational setup at Apple has had visible effects on this version it would be worth showing off what the Ive-and-Fedherigi future is going to look like, but of course it may be far too soon for us to expect any dramatic new directions to be ready.
