The current Internet Draft for IPv6 Addressing Architecture has a couple of items worth noting. Section 2.5.1 requires that interface IDs be 64 bits long, effectively deprecating any prefix longer than a /64. (Don’t let the reference to “modified EUI-64″ scare you - hand crafted addresses are still fine.)
Later on, section 2.6.1 gives a special meaning to addresses where all the bits to the right hand side of the address are zero.
Some of this is already present in RFC3513. In any case, should this draft make it to RFC status with these sections intact, it will break the practice of using long prefixes such as /126 and /127 for point-to-point links between routers. So while we’ve been insisting that using /64s for all subnets is a good idea, it may well become required practice in the near future.