I found an interesting read about the different threats found in IPv4 compared to IPv6. This might be worth your time (:

http://www.nanog.org/mtg-0405/pdf/miller.pdf

Sad to hear that MitM attacks will be pretty much as easy as before. ARP really should be revised in my opinion .
edit: hmm, but arp has nothing to do with network layer 3, so IPv6 has nothing to do with that. How smart of me :P