Nintendo recently announced that Pokémon Ruby/Sapphire for GameBoy Advance is not backwards compatible with previous Pokémon titles.
Unlike earlier installments, Pokémon Ruby/Sapphire does not allow Pokémon trading or battling via link cable with GameBoy and GameBoy Color Pokémon games. In Pokémon Gold, Silver, and Crystal, trading was allowed with Pokémon Blue, Red, and Yellow through the means of a time-traveling device. Pokémon found only in Gold, Silver, and Crystal could not transfer to Red, Blue, and Yellow for obvious reasons, but Pokémon found in both games could be easily shared. Since Pokémon Ruby/Sapphire lacks this capability, trainers will have to start from scratch instead of bringing in tried-and-true Pokémon via trade from previously cleared games. In addition, Pokémon Ruby/Sapphire is not compatible with Pokémon Stadium and Pokémon Stadium 2.
Pokémon Ruby/Sapphire is set to be released later this month in Japan. A U.S. release date has not been announced, but considering Nintendo's track record with this series, one is to be anticipated.