Well, the future will tell us, but for the moment, Anarki is a non-official experiment around the "real" Arc, and as far as I know none of the propositions made in Anarki can be found in the official versions (it's very early for that, I know). On the contrary, debian stable & unstable are both official and the vocation of unstable is to become the next stable version.
"as far as I know none of the propositions made in Anarki can be found in the official versions"
There are a few bug fixes that appeared first in Anarki, were brought up on the forums, and ended up being added to arc1 or arc2. But you're right, I don't believe PG pulls changes from the repo directly.
I'm no lawyer and I haven't read the artistic license very closely, but Anarki's under the same license as the original codebase, so I'm pretty sure there shouldn't be a problem...
I understand your point, but I am sure you'll agree that Paul definitely will accept a feature if it is really cool. Thereby Anarki is de facto Arc Unstable.