The only thing approaching Twitter spam is *NOT* people you have chosen to follow who are ranting or Retweeting. etc, it's people @reply to you with stuff that you don't care about.

Things like Twitterhawk are going to make this much more common, where people can automatically search for "buzzwords" and then send automatic @replies.

Or you get someone with too much free time who decides it would be funny to create a bunch of Twitter accounts and have them all @reply you.

Both of these things have happened already, just on a small scale.

At which point you can complain to the @spam account, but like every other kind of spam, by the time you complain about it, even if you get the account closed, it's too late, it's already taken your time and attention.

That's the only thing you can legitimately call Twitter spam.