Bring a GBA with your favorite Pokemon game to your local Toys 'R' Us and they will trade you a Mew.
To celebrate the release of the new Pokemon movie 'Lucario and the Mystery of Mew', Nintendo and Toys 'R' Us teamed up to hold a special event at Toys 'R' Us stores across the US. The event takes place between noon and 3pm tomorrow, September 30th.
In order to receive your Mew, make sure you bring a Game Boy Advance or Game Boy Advance SP (Game Boy Micros will probably not work) with a copy of one of the following Pokemon games.
It seems as though it will work like a traditional Pokemon trade using the Game Boy Advance Link Cable. Therefore you must have a Pokemon you are willing to lose in your party at the time of the trade (You better go catch a magikarp!). Also, make sure your game is saved at the top floor of a Pokemon center to save time.
If you want to get Mew on your Pokemon Emerald game, you must have the National Pokedex, which you receive after beating the Elite Four.
This is one of the rare opportunities to get Mew legitimately in the US. Don't miss it!