If you use an attack that puts your opponent's Pokémon to Sleep (like Haunter's Nightmare), and the Defending Pokémon was already Asleep before you attacked, do you still have to flip a coin after the attack to see if it wakes up?
Yes, they get to flip at the end of each player's turn.
Source: WotC Chat (2000-02-10)
If one of your Pokémon was Poisoned, and a few turns later it is KO'd by the Poison, does your opponent take a Prize card although the Poison KO'd the Pokémon, and not your opponents Pokémon?
Yes, he would still draw a Prize card. Whenever a Pokémon is Knocked Out, the other player gets Prize cards -- even if you Knock Out your own.
Source: WotC Chat (2000-02-10)
Can the Switch card be used to retreat a Paralyzed, Confused, or Sleeping Pokémon?
Yes, you can use a Switch card to retreat a Confused, Asleep or Paralyzed Pokémon.
Source: WotC Chat (2000-02-03)
Does Magmar's Smokescreen effect end if the Defending Pokémon evolves?
Since Magmar's Smokescreen attack references the Defending Pokémon, by it evolving, it removes that effect.
Source: WotC Chat (2000-02-03)
Does Fearow's Agility attack block Kangaskhan's Fetch?
Actually it does not; Fearow's Agility says stops damage done to Fearow, and the Fetch just draws a card. This is the new ruling from R&D.
Source: WotC Chat (2000-02-03)
When you Knock Out more than one of your opponent's Pokémon in a turn (i.e. using Selfdestruct or Raichu's Gigashock attack) do you receive a Prize for each Knocked Out enemy Pokémon?
Yes you do. And if you knock out some of your own, your opponent gets to draw prizes as well.
Source: WotC Chat (2000-01-27)
Are you allowed to have as many DCE as you want in a deck like normal Energy or only four like any other card?
Only 4 maximum, it is NOT a basic Energy card (the only type you can have more than 4 of in your deck).
Source: WotC Chat (2000-01-27)
Are Promos legal in tourney and league play?
Promo cards are legal in tourney and league play [As long as they have been released in the local area].
Source: WotC Chat (2000-01-27)
Do you have to attack if you have enough Energy cards attached to your Pokémon?
No, you always have the choice whether to attack on your turn or not.
Source: WotC Chat (2000-01-27)