Problem
Trying to use the assert_difference with Rails 2.1.0 produces errors if trying to use it with the syntax of another rails version.

Solution
The correct syntax for using assert_difference with rails 2.1.0 is:

def test_should_create_user

assert_difference(User, :count, 1) do

user = create_user

assert !user.new_record?, "#{user.errors.full_messages.to_sentence}"

end

end