JoelWhy, I agree it is in use as a proper name.
The question is the correct use. Which probably boils down to personal preference.
I'd happily use 'God' in the following circumstances:
1) The existence of one God is proven. In that case, I'd also go for all caps: GOD
2) All other gods new and old are forgotten and Allah (or anything else) has a cultural victory (so spread Stargate and Valhall copies far and wide).
3) I believe there is one God.
4) In a fictional setting.
As long as we have doubt and such things as 'a god' and 'gods', we can argue against automatic capitalization.
Obviously, Jahweh insists on 'God', but as long as he is as plausible as any other god, I won't give that particular god any preferential treatment.
'God' is a label and we have plenty of gods - the name for one of them is Huitzilopochtli.
'Devil' is a label for numerous evil forces (gods really once you look at history and powers) - some names are Satan, Belzebub, Lucifer Morningstar etc.
Yes, I write Pinocchio the same way - that is a specific name, but I won't normally capitalize 'Wood Doll'