about that giveaway site posted on the previous page, a few notes:
1. the pokemon on that site ARE legit, even if you are obviously not getting them in a legit way. They are the pkmn files generated immediately after the event/pokemon is received over ni-fi/wi-fi.
2. As long as you get the event pokemon that way you are fine. However, please note that you MUST have THE EVENT pokemon in your party to correctly trigger the event in B&W. Just trading up a celebi from Ru/Sa/Em for example isn't good enough. It has to the the "fateful event" Celebi (same thing with the GSTOP shinies).
3. Arceus was only available directly as a mystery gift (in the US at least) in 2009. I would hope that they would offer him again sometime this year. The flute that was supposed to be used to get him was never officially distributed.
Now one interesting thing is this.. IF you are ok with doing the above "illegal" method (as far as official distribution goes), a slightly more fun way to "illegally" obtain the event pokemon and items is to grab the actual distribution ROMs from the net, throw them on a flash card and turn a spare DS into an actual Nintendo Event device. Don't ask me where, how, PM me or anything.. but... if you want to be as close to legit as possible, this seems like a fun way to do it. You just setup the one DS with the distribution ROM you are looking for and then do the mystery gift thing the normal way.
and as a bonus fun fact, you CAN "legitimately" get the Flute for Arceus this way as well. Apparently a distribution ROM for the flute was in fact created, but was never offered to the public (weird).
With all of that being said, going to that website seems like a 1000 times easier way to go about getting the event pokemon.