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.