MPLS as well as the IETF softwires techniques (the MPLS model without using MPLS i.e. tunnel from ingress to egress via automated setup tunnels - gre, l2tp, or native IPv4 or IPv6) can or will shortly be able to be used to tunnel IPv6 over IPv4 or vice versa. softwires in effect treats the non-native core infrastructure as an NBMA layer 2.
The advantage of these techniques verses dual stack is that they push the complexity of dual stack to the network ingress and egress devices.
Dual stack isn't all that complicated, however when you think about running two forwarded protocols, two routing protocols or an integrated one supporting two forwarded protocols, having two forwarding topologies that may not match in the case of dual routing protocols, and having two sets of troubleshooing methods and tools, I think the simplicity of having a single core network forwarding protocol and tunnelling everyting else over it becomes really attractive.
huh? and your tunnels do not have *worse* congruency problems than dual stack? gimme a break. randy