You will be able to get Mew on Ruby, Saphire, Leaf Green, Fire Red or Emerald.
A direct link to the news story is Here.
There is also information on Pokemon.com about getting Mew:
Just make your way to the video game section of any Toys"R" Us near you on September 30, between the hours of 12 noon and 3pm, for your chance to trade Mew onto your team. You'll have to trade a Pokemon in order to get Mew. so make sure to have one on your team that you don't mind parting with - but it's well worth it! But we have to warn you - Mew is still a rare and elusive Pokemon, so trades of Mew will be limited and only while supplies last!