Security of DNSBL spam block systems

["Big_Bandwidth" <big_bandwidth () hotmail com>]

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