I thoguht I already posted something like this here... guess not...
Anyways, I have to type this really fast, so please forgice any mistakes I may have. The Mew you receive from the download will be one of two variations, and special attacking one with moves like Thunderbolt and Ice Beam, and a physical attacking one with moves like Shadow Ball and Explosion. Apparently, the Mew will be holding either a Leichi berry or a Petaya berry(it corresponds with what type of mew it is), and the physical attacking Mew will have an Adamant nature and the sp. attacking Mew will have a Modest or Timid nature. The last two things I told you are unconfirmed, so take them with a grain of salt.
No, I didn't copy this from some official website, I just tried to sum up everything I know about the Mew download as fast as possible. Too bad we don't get the Old Sea Chart, or else I woulda been SRing for Mew all day long. Does anyone think you will need the National Dex to get the Mew downloaded to your game(remember at JAA that you didn't need to have the National Dex to get Celebi and friends downloaded to your games.
Also, I haven't gotten Rayquazaz yet. Wish me luck guys.