The Viewty is a imaging-oriented mobile phone. Its primary purpose actually happens to be its still and video abilities. If Nokia were going for the same goals within the N900, then you would have seen it marketed in the same light. Nokia isn't.

So the question shouldn't be why doesn't the N900 do this, but what business sense would it make for it to do this in light of another niche product not in its class of device that does it?