Last year's McColo takedown gave hope that shutting down providers would help shut down spam, but no such luck, long-term anyway.
Taking down the botnets' hosts doesn't even slow their spew for long: Cutwail's got axed a week ago with the takeout of a Latvian server, but was back up to speed within a matter of days.
Botnet efficiency and their self-defensive capacities seem likely to grow, and grow exponentially, in the future.
With that in mind, tightening your own defenses and mail filters is a key protective tool for your business, as is making sure that none of your equipment is working with the botnets instead of against them.