Pokémon HeartGold and SoulSilver was released on 12 September in Japan and 14 March in North America.

Important notes
The Pokédex Count is by Capture, not by Seeing.
Bellsprout tradeable for Onix in Violet City.
Can't go inside the gym until Sprout Tower.
The Pokédex has a Form function but now you can compare them.
Get radio card before going to the gym.

Outer Appearances
1) The Clefairy(s) on Monday nights - Mt. Moon
Gym Leaders
Falkner: Monday - Celadon Dept. Store 4F
Bugsy: Thursday Daytime - Viridian Forest/Friday Daytime - National Park
Whitney: Saturday Daytime - Goldenrod Dept. Store (6F)
Morty: Monday Daytime - Outside Tin Tower
Jasmine: Early Afternoons - Olivine Diner
Pryce: Saturday Morning - Lake of Rage
Clair: Saturday Morning - Dragon's Den
Brock: Afternoons - Diglett Cave
Misty: Evenings - Route 25
Janine: Evenings - Reception Gate
Sabrina: Friday nights - Olivine City Harbor
Blaine: Tuesday Morning - Cinnabar Island
Maylene (Veilstone City) - Celadon City
Crasher Wake- Celadon Department Store (Giving Sinnoh Starter Accesories)

As the game is close by, Serebii.net, 2ch.net has updated lots of news.

1) Gym Leaders have new Pokémon.
2) Legendary Pokémon,
http://www.serebii.net/heartgoldsoulsilver/legends.shtml
3) Starter Pokémon
-Johto's: Start of game
-Kanto's: 16 badges
-Hoenn's: Defeating Red and meet man in Silph Co.
4) Miscellaneous
a) Pal Park is in Fuchsia City (over Safari Zone)
b) Trade from Diamond, Pearl & Platinum after getting Pokédex.
c) Snorlax is findable outside Diglett's Cave again at Level 50.
d) Red is findable in Silver Cave again, with different Pokémon
5) Shiny Pokémon following you, will be shiny when outside their PokéBall.
6) Team Rocket
a) A new Rocket Executive in Mahogany Hideout, called Lambda.
b) A female Executiver is also there in a double battle, teamed with Lance against her, called Athena.
c) In Goldenrod, you battle all the executives again and then a final executive, Apollo and he has a Houndour (Lv. 35), Koffing (Lv. 35) & Houndoom (Lv. 38).

First, the female trainer's name will be Kotone (コトネ).
They also hint that the special Pikachu in the Yellow Forest PokéWalker course knows Fly.

New Pokéthlon events were revealed.
The first is Catch Saucer and involves you controlling your Pokémon against your opponent to try and grab flying objects being shot across the screens.

The second is called Steel Plug and features you controlling your Pokémon around a course trying to get as many pegs as possible.

Added version of the previous post
The first trailer confirms the existence of the Wi-Fi Plaza in the games. Connecting to other players via Wi-Fi will also appear as similar to Platinum.
The trailer also shows the battle sprite of the Gym Leader, Jasmine.
Similar to the ones in Platinum, it is animated.
The Gym in Violet City is also seen with more elevation.

The second video shows that the Headbuttable Trees from GS will be back.
It seems that any tree is headbuttable. It is unknown if they all give Pokémon.
The second floor of the Johto Radio Tower is also shown.
The battle sprite for the legendary Suicune is also seen.
Lastly, the rare Munchlax will be available for capture in the PokéWalker.

The official site updated with a variety of new pictures.
However, in addition to this, it also confirms the return of the
Global Trade Station and the WiFi Plaza, same as Platinum.
Also, the Battle Frontier is in Johto.
The facilities and Frontier Brains are the same as Platinum.

First, Apricorns will be coming back.
However now, you have the ability to grow them yourself,
Using the SquirtBottle to grow them.
They're stored in an item called the Apricorn Case.
They can be given to Kurt in Azalea Town to have some special Poké Balls.
Here are their uses:
Fast Ball: Captures Pokémon that Flee
Friend Ball: Increases Happiness of Captured Pokémon
Heavy Ball: Captures Heavier Pokémon more successfully
Level Ball: Captures Pokémon of lower levels easier
Love Ball: Captures opposite gendered Pokémon easier
Lure Ball: Captures Pokémon caught with the Fishing Rod easier
Moon Ball: Captures Pokémon that evolve with the Moon Stone easier

-The Mystery Egg subset will also make a return.
The Pokémon inside the egg, however, has yet to be confirmed.
(It is unknown if the Egg you receive will still be Togepi.)
-Next, it reiterates Kanto's return in HeartGold & SoulSilver.
-It also confirms the National Park's Bug Catching Contest making a return.
This contest features you having to capture the best Bug Pokémon you can.
-It also confirms that the new area west of Cianwood City is a Safari Zone.
-And also, there is a new Battle Frontier style facility to the west of Olivine City.
-Rayquaza is catchable later in the game at Level 50, requiring Kyogre & Groudon.
-And Groudon and Kyogre are available, confirmed to appear in SoulSilver/HeartGold

Edit @ 10 August

-Apricorns also can convert into juice to boost your Pokémon's Pokéthlon stats.
-Three new Pokéthlon events have been revealed:
= Break Block (Your Pokémon breaking blocks using their hand, using the Touch Screen.
= Change Relay, also controlled on the Touch Screen.
= Push Circle. You control all your Pokémon at once

It also shows that you can customise the areas within the Safari Zone. Moving areas for quicker access should you need it to allow for easy access to areas if you're looking for certain Pokémon.

-There is another two Pokéthlon events.
It appears that you'll have your three Pokémon across from another three Pokémon. This is essentially a Snowball Fight called Shooting Snow. Another Pokéthlon mode is called Ring Out Fight where your 3 Pokémon have to knock the other three Pokémon out.
-PokéGear will be in games, appearing in the Menu as usual.
It contains the same features as before specifically the Phone, Map, Radio & now the Itemfinder. -It also shows the capturing of Pokémon on PokéWalk.
-You can also register two key items in this game and they are accessible via the Touch Screen.