[akhtene]

Perhaps from the start Saruman didn't intend to side with anyone, as he wanted power (and the Ring as the incarnation of power) for himself. But while using the Palantir of Orthanc he got in too close a contact with Sauron and got trapped by him. Thus some of his actions were inspired and guided by Sauron, though I don't think Saruman was completely overpowered (the way Denethor was on similar occasion). I don't believe he ever really wilingly sided with Sauron, only for his own convenience or under control; I think it was ultimate power he was after.
But he definitely betrayed the White Council and the case he had been sent to assist.