pokemon actually confirmed this.
When Arceus was originally released, it was pronounced Ar-Sea-Us, as that's how Kenji Okobo, CEO of Pokemon International, pronounced it when Diamond and Pearl came out. However this caused a problem because Ar-Sea-Us sounded like Arce, which is how the british say ass. So in 2011, when the Pokemon International started to dub the English version of the anime, they called it Ar-Kee-Us, and this is confirmed official because it was dubbed by Pokemon International. Prior to 2011 the anime was dubbed by 4Kids, so the pronunciations weren't official. We can also know that Ar-Kee-Us is official because of Silvally. Silvally's ability is RKS system, a pun on Arceus. if it was pronounced Ar-Sea-Us, the ability would be RCS System.
