Importers of Heart Gold/Soul Silver listen up! The event for the 10th Anniversary Mew started 11.11.09, and ends on 11.23.09, so grab it while you can!
There's nothing special about the Level 5 Mew, other than the Premier Ribbon that it has. Since it's level 5, the only attack that it knows is Pound, but it will (obviously) learn all of it's other moves as it levels up!
This event is repeated for Japanese Diamond, Pearl and Platinum owners from 01.29.10 to 02.11.10, so if you happen to own one of those be sure not to miss the second download!
Remember, this is Japan only!
Stats and stuff:
Another reminder: 5 days after this event (Nov. 27th) Heart Gold/Soul Silver owners will be able to download the Enigma Crystal. When taken to Pewter Museum it will be recognised as Soul Dew, and later Steven Stone will show up to tell you that it represents Latios/Latias (depending on your version). Afterwards you will be able to find Latios (Heart Gold) and Latias (Soul Silver). This way you can get both Lati's using one version!