An update from Famitsu on Heart Gold/Soul Silver:
http://bulbanews.bulbagarden.net/wiki/File:Famitsu_HGSS.jpg
Not a lot as of yet, but a 3D starter select screen can be seen, as well as the starter Pokemon following the trainer. Also, a notable screenshot at the bottom-right corner of the second page shows an in-game event from HG/SS involving the Pichu received from the Arceus Movie event.
On a side note, the Movie Event Arceus is said to have some in-game effects when transferred to Platinum, possibly with regards to the overall plot of the game.
Overall, to get back on topic, HG/SS is looking like it's well in development already.