After many, many years (and many griping wolf fans) Forge World has finally produced Space Wolf upgrade kits for Horus Heresy. They've got wolfy torsos and wolfy shoulders in mkII, mkIII, and mkIV varieties. If you've been waiting until now to start building your Space Wolf legion, now you can begin... at least from a model perspective! We're still not sure how the Space Wolf rules will actually work, so be careful! Rumor has it that they (like the Thousand Sons) will be very different from the legions we're used to.
I do think it's odd that we don't have any Space Wolf-specific heads... perhaps those are in the works?
In any case, I'm not a Space Wolf player (though back in '99 I almost was...), so my interest is minimal. But if you are a Space Wolf player, I'm happy for you... now go build a legion!