Mew was available in Japan through a wi-fi event, so it's possible you may be able to get Mew this way. (HeartGold/SoulSilver)
In older games like Emerald in the related question, the events are all real-life events that they held when the games were out. They don't do events for older games but they do for newer ones so you may be able to get it for D/P/P or HG/SS. Check the official site pokemon.com for announcements.
EDIT: Mew event running from October 15-30 2010!
http://pokemondb.net/news/new-wi-fi-event-to-obtain-mew
