As with most things, it's not a black/white thing.
If, as mentioned, you use multiaccounts to exploit things like the referral system, etc., then yes, it's very bannable.
However, the multiaccounting policy regarding ban evasion is fairly relaxed, depending on what you were originally banned for. This is because we consider the loss of items/TC/whatever due to the ban fair enough punishment.
EDIT: Damn you skulfuk, you've beaten me!