So far, there is no Nintendo event to get Arceus for your DS or DSi.
in 2010
It is from an Nintendo Event.
In Japan in December 2010
there was none that i know of...
Official events are the only way to catch Arceus.
You have to attend a official Nintendo event. For now Arceus has been released only in Japan.
you need the action replay or a Nintendo event.
Nope.
Yes, but you have to get it from a Nintendo event (If they have any new events for Arceus) or trade with someone who got it from an older event.
There is no real way of telling whether or not nintendo will put out an event for a specific pokemon. I would love it if there was an Arceus event in 2013 because I could really use an Arceus myself. But, like I said we won't know until nintendo announces it. If I had to take a guess I would say yes and probably sometime closer to the end of the year.
Um, WHAT? Anyway, Arceus can be caught in the Hall of Origin (available by Nintendo Event).