It doesn't really fall under the definition of an escrow:

    a bond, deed, or other document kept in the custody
    of a third party and taking effect only when a specified
    condition has been fulfilled.

    [usually as modifier] a deposit or fund held in trust or
    as a security

The arbitrator gets "voting power" in regards to deciding where the funds goes, he doesn't keep it in his custody nor he have full control over it. What Bitcoin allows to do is really unique, and was never possible before. When the money is in a multisig address, it isn't really "owned" by anyone.

Edit: perhaps you could say that its owned collectively by the Bitcoin network, that can decide to allow moving it elsewhere if a transaction that fulfills the Bitcoin rules is received. Under that definition, one could say that the funds are in an escrow controlled by the Bitcoin network as a whole.