Update: Thanks to groudon & Ryan for pointing out that the day you’ll be able to get Mew for your GBA Pokemon games is Saturday September 30th 2006 at Toys R Us.
To celebrate the release of Pokemon: Lucario & The Mystery of Mew, which is coming to DVD on September 19th, as well as the launch of the Pokemon Mystery Dungeon games on the 18th. Toys R Us stores all over America for one day will be distributing the mysterious Pokemon Mew.
Bring one of the following Pokemon game paks to a participating store: Ruby, Sapphire, Emerald, FireRed, or LeafGreen, to receive Mew. – Via Gonintendo