Spammers have and are developing several techniques and technologies over the years to spread their messages, and bypass anti-spam filters. One of these technologies is a type of program called a 'spam zombie'. These zombies have become more and more powerful over the years, and now they're capable of making RBLs (aka Blacklists) useless.
The new Zombie architecture, acts as proxy to sends spam upstream via the ISP's mail server. This prevents RBLs from working properly, because it is impossible to block all the possible addresses where the messages can come from.
As you're probably already aware, the spam problem isn't getting better. The SpamHaus Project believes that spam is going to skyrocket new levels. The total amount of spam is expected to grow to over 90% in 2005.