Just my take on Arc...
Sony already had a motion controller 'SixAxis', and they did Nothing with it, ya a game or two, and then some games with a motion notion here and there but that was it.
When Sony first talked about the Sixaxis, they made it seem that many, many, games would have the feature and it would take the place of rumble. Now it's like they want people to forget about it.
You don't really see Sony trying to push the 'Arc' like other companies are pushing their versions, it seems more like they are taking a 'if it works it works, if not o well' approach.
I went to allot of Game stores in my area these past couple days, and majority of the discussion were about new games just released, Natal, DS:XL, Ipad, but nobody even mentioned Arc.
PS3 never really grabbed the 'family/casual' approach with the PS3, people touted about how majority of PS3 owners are adults, just look at the games recently released, nothing really says 'family console', and to me Arc seems to like a Wii approach to gaming, but with an Adult non-casual install base.
This is just my view on Arc
Personally I have never seen nor heard anyone (outside of this site) talking about Natal, or the Arc. Even the friends I have who work in the game industry have never mentioned either. If one's going to fail, then both probably are.