[jchavannes]

And how hard would it be for an attacker to use this logic?

  if crawler == twitter
    normal website
  else
    insert virus/spam
  fi

I am strongly averse to redirector links because of the security risk. You can't trust the destination.

[shawnz]

I don't think it is an automated process. Rather, when there's an outbreak, twitter uses their control over t.co to contain it.