In the bottom right corner of page 97 of October's issue of Nintendo Power is an annoucement that on September 30th you will be able to download Mew to any of your GBA Pokémon games (Ruby, Sapphire, LeafGreen, FireRed and Emerald) at any Toys R Us location. This is supposed to be a promotion commemorating the release of the movie Pokémon: Lucario & The Mystery of Mew and Pokémon Mystery Dungeon.
However, I work at a Toys R Us and have not heard any details yet. I also talked to our region's Nintendo rep and he hadn't heard anything yet. I also checked both www.nintendo.com and www.pokemon.com and there was no mention of it, however it is mentioned in this wikipedia article on Mew http://en.wikipedia.org/wiki/Mew_(Pok%C3%A9mon).
This is going to be monumental to Pokémon fans. The first legitimate release of Mew in America. I wish I had more confirmation about it. But if it's in Nintendo Power, it must be true. All's I know is that I BEST be working on the 30th. I hate wasting gas to drive up there when I'm not working (although it would be worth it for Mew).
I'll post more info (unless someone else beats me to it, which is highly likely what with my lack of time online these days) when I hear it.