Here is an interesting and informative talk given by a Systems Administrators about BGP prefix hijacking and how spammers use it to evade RBL's. Here is an excerpt from the description:

BGP prefix hijacks take the IP addresses of others and make them your own. This talk provides a chilling account of the current use of prefix hijacks by spammers in a successful effort to defeat RBLs. Placed within the context of the history of the spam wars, this talk makes clear the grim future we face if we continue to escalate the spam wars into the network layer; namely, a future where every spammer on earth can arbitrarily choose and make routable an unallocated ipv4 address (one that the RBLs have never seen) once per day for the next few hundred years or so without ever using the same address twice or ever colliding with any other spammer.

It's from the Lisa '07 Conference that took place in Dallas this past November. Watch and learn!