Bill Manning;
% Expect to see routers being optimized that will only route
% the upper 64bits of the address, so you might not want to do
% anything smaller than that.
This, if it happens, will be exactly opposed to
the IPv6 design goal, which was to discourage/prohibit
hardware/software designers from making presumptions or
assumptions about the size of prefixes and HARDCODING them
into products.
That is a foolish design goal demonstrating lack of understanding
on how large 2^64 space is.
A better design, for example, is to use only upper 16 bits (note
that 100,000 routes of IPv4 is already overkill) at the core
backbone, which makes high speed backbone routers not very
expensive and faster.
Though multicast and QoS needs more bits, they, anyway, need
separate mechanism.
Masataka Ohta