The web has started commenting on twitter's decision to limit the number of accounts that a given user can follow. Having a hard limit is a smart move for multiple reasons. Not only does it allow you to more finely bound the computational load of the message passing architecture, it negatively impacts only two groups, namely spammers and the obsessive-compulsive.
This is a good first step that I have pointed out in an interview once before. I suspect that Twitter will also be working on a throttling policy as well as an IP and content blacklisting technology as follow-on mechanisms to continue to battle spam.