Just because information hasn't been released, doesn't mean D/P aren't close to being finished. Hell, it may be completely done and merely being revised at the moment.
Of course there isn't going to be any information on D/P right now, not with Pokemon Ranger 'taking the world by storm'. One game at a time people, one game at a time.
Though if memory does serve me correctly, the first pictures of R/S were released sometime in March and it was actually released in November of the same year [meaning, we sure as hell should get some pictures soon]. The point is, it takes time to release a game. You need to have the right marketing tactics by giving the public a little bit so they want more.
On the Sevii Islands, yes, I can see it being used as a last resort. However, who says Brandon is coming up within the next few episodes? And after that, what about the League for the Battle Frontier, and May's Grand Festival? Will that not take up a couple of months? Slap some fillers in between all those main events, and you're pretty much done right there.