aaargh.. I feel so stupid now..
been looking through show-run at least ten times.. and always overlooked that one line that pointed the ipv4 default route to my old ISP router from the former network config.. as soon as I fixed that entry, I got "line protocol on Tunnel0 changed state to UP"..
seems to confirm that I'm running fine via v6.
now the only thing left to do is secure that tunnel a bit..
got any suggestions for a "block all inbound, permit all outbound" config?
I'm not too good with IOS access lists.. but that should be sufficient for now.