I could live with something like: my greylisting server whitelisted x
IP addresses (I'm figure John Levine could give us this number quite
quickly for his a "several hundred individual mailboxes, a few dozen
mailing lists, and the abuse.net message forwarding system.")
It's about 22,000 addresses. My fixed greylist covers about 3000 more
addresses in CIDR blocks, of which probably no more than a few hundred
send mail.
I have no idea how this relates to the pattern a larger ISP would see.
R's,
John