Notice this from Paul Dini's twitter:
Passed on a big Batman project this week to do a smaller, more personal one. No worries, Arkham II still a go.
Wonder what that big project is, he did say he's smaller one has Batman tho, and i still do wonder whats with the guest-writers in SoG. =p I guess he really is still on writing Arkham II if he says "still a go" and not like "yeah already done with that".