Security of DNSBL spam block systems

What are the security implications of someone hacking a DNSBL (Real-time-spam-block-list) and changing the block list to include (deny email from) some very large portion or all IPv4 space?

Given that a signifigant number of the spam blocking lists seem to operate on a shoestring budget in someone’s basement, how can we be assured that they have sufficient resources to secure their systems adequatley, and monitor for intrusion 24x7?

Unless I am missing something, this would seem to be a real handy and centralized method for someone to interfere substantially with the proper operation of a few thousand email servers and hold up global email traffic for a few hours.

-BB

IMHO Even the really large DNSBL’s are barely used – I think (much) less than 5% of total human mail recipients are behind a mailserver that uses one…
–Phil