A fairly short article here, although I do believe that the new, official website for HeartGold/SoulSilver speaks for itself here. Even though it's all in Japanese, the website features numerous screenshots of the actual game, showing the player characters interacting with each other, a screenshot of the player battling the rival character, event screenshots showing Cynthia with Arceus as well as the Notched-Ear Pichu event. All these screenshots and more can be found here:
Again, the website is entirely in Japanese, but the number of screenshots on here are more than enough to make this well worth checking out. The website will also be updated frequently as more stuff is released regarding the two games.
UPDATE: PKMN.NET has confirmed that the Kanto region will be making a return in HG/SS. The odds of any other regions being included now are very unlikely, with the exception of a possible, brief Sinnoh appearance tying in with the Arceus event.