Well the legendary birds being available wouldn't be all that surprising. But if Platinum and HG/SS are supposed to be in a similar time frame then that would not happen. I guess, we'll see. The official site has not mentioned the legendary birds, so we'll wait for them or the game itself (PPRE script decompiler will know in instants whether they are available)
As for Distortion World, its pretty safe to say that they signature pokemon's world will stay exclusive to platinum. Now if they make Giratina available in HG/SS then they would stick Distortion world in some where (with a some, distortion world got larger after the disruptions caused by team galactic, logic)