Has anyone been looking at the full headers of their SPAM lately? At least half of it is coming from DSL and Cable Modem systems here in the U.S. Mostly from Comcast, RR, and Adelphia. At some point these companies will have to stop their customers from SPAMing the world.
Most likely open relays rather than the actual owner doing it. I get around 20 attempts a day on my mail server from people looking to see if they can relay from it.