Again? I love the discussion on Arceus, but...
Arceus was definitely born at the very beginning, "at the heart of chaos, where all things became one, [there] appeared an Egg. Having tumbled from the vortex, the Egg gave rise to the Original One [(Arceus)]."
From itself, Arceus made Dialga and Palkia, which made time flow and space expand. Then it made the Lake Trio. Then "the two beings wished, and from them, matter came to be. The three living things wished, and from them, spirit came to be."
After this, Arceus went to sleep. This is where speculation begins.
Now, perhaps the idea of Mew was made before Satoshi Tajiri thought up the idea of Arceus, so there may be faults there. However, I'd think he'd have this all planned out beforehand, so maybe he wants there to be that ounce of confusion - it makes for great discussion. Perhaps he's leaving it up to the fans to decide for themselves.
In the end... FACT: No Pokédex entry states that Mew IS the ancestor of all Pokémon or that it made Arceus, just that "...some people began research to see if it is the ancestor of all Pokémon" (Crystal entry) and "...many scientists believe Mew to be the ancestor of Pokémon" (D/P entry). Legends state Arceus was born before the universe existed, and that it created Time, Space, Emotion, Willpower, and Knowledge (and, oddly, "it is described... as the Pokémon that shaped the universe with its 1,000 arms" [Diamond Entry]. This may be explained in Movie #12, though.)
I guess I get the award for going into too much depth on what Pimpwalkin' Mateo Johnson said, though. xD