I'm figuring 5th gen would come out fall 2010 in Japan since new generations come out every 3 to 4 years roughly. Diamond came out September 2006, so 2010 seems likely for a Japanese release. We'll probably start getting pokemon leaks soon, and this notched-ear Pichu could even have something to do with it. Attention was brought to the fact that its female, and it was kept hidden until Coro-Coro despite not really having anything to hide. So they could be adding more gender differences, old pokemon included, or possibly new markings. There's also going to be yet another Mystery Dungeon game announced in January, and some 4th gen pokemon appeared as statues in the first Mystery Dungeon games.
So, yeah, I think there could be news as early as January's Coro-Coro. There's apparently some kind of pokemon festival in January too, though I can't remember what it is...