> class c nets, which are 2/3 the total.

Why can' the NIC reallocat the old class c's and use CIDR?

They are trying. But you can only lead this horse to water -- we can't take
prefixes away, we can only make them harder or more expensive to get routing
for. This is a lengthy process, and its success is the only reason we still
have a 30,000 element core routing table -- endpoint growth never stopped, but
new prefixes are all fairly short, and there has been a nearly corresponding
drop in old (long, expensive) prefixes in recent months. As Randy pointed out,
though, we've probably about squoze all the water out of the old classC sponge.