Mass-generating fake Twitter accounts for profit

Twitter is a great platform for these spammers:

* Can reach a large amount of users by mentioning popular events (like trying to get high page ranks on google for spam pages). * Users expect short (and not fully descriptive) messages with a link. * The link is shortened to obfuscate where you are really going. * No spam filtering yet (Is there even enough data given in 140 characters to make effective spam filtering?).