[Boromir88]

Originally Posted by Inziladun

Saruman was under Sauron's domination, in that he no longer opposed Sauron's victory, and was in fact supporting it, assuming he could not get the Ring for himself.

I'm assuming you are referring to this from UT:

It may be noted that the effects were different. Saruman fell under the domination of Sauron and desired his victory, or no longer opposed it. Denethor remained steadfast in his rejection of Sauron, but was made to believe that his victory was inevitable, and so fell into despair. The reasons for this difference were no doubt that in the first place Denethor was a man of great strength of will, and maintained the integrity of his personality until the final blow of the (apparently) mortal wound of his only surviving son. He was proud, but this was by no means merely personal: he loved Gondor and its people, and deemed himself appointed by destiny to lead them in this desperate time.

Saruman no longer opposed Sauron, which is of course a lot different from Denethor who always maintained his "integrity." However, I would not call Saruman dominated by Sauron, in the puppet-sense that the movies protrayed. Chris Lee was correct in one of his many interviews where he claimed that Saruman sought the Ring for himself, and desired to be the Ring-lord. In Saruman's mind, it would be easier to achieve his own goals if he joined with Sauron, planning in secret to back-stab him, when the proper time came. When Saruman captures Gandalf he reveals his true mind of joining Sauron, becoming his trusted friend, and then attempt to boot him out of Barad-dur. But I think the UT has a better quote to reveal Saruman's true intentions:

Two days after Gandalf had departed from Orthanc, the Lord of Morgul halted before the Gate of Isengard. Then Saruman, already filled with wrath and fear by the escape of Gandalf, perceived the peril of standing between two enemies, a known traitor to both. His dread was great, for his hope of deceiving Sauron, or at least of receiving his favour in victory, was utterly lost....Therefor to his [The Wiki's] challenge and demands he received only the answer of the Voice of Saruman, that spoke by some art as though it came from the Gate itself.

"It is not a land that you look for," it said. "I know what you seek, though you do not name it. I have it not, as surely its servants perceive without telling; for if I had it, then you would bow before me and call me Lord. And if I knew where this thing was hid, I should not be here, but long gone before you take it. There is one only whom I guess to have this knoweldge: Mithrandir, enemy of Sauron..."

...Such was still the power of the voice of Saruman that even the Lord of the Nazgul did not question what it said, whether it was false or short of full truth; but straightway he rode from the Gate and began to hunt for Gandalf in Rohan.~Hunt for the Ring

Saruman basically told the Witch-King if he ever got a hold of the Ring the Wiki would bow to him, yet Saruman was such a sweet talker, Sauron did not discover Saruman's betrayal until much later. In The Uruk-hai chapter Grishnakh says that Saruman is a "treacherous fool" and the "Eye is on him," but I still would not say Sauron was certain of Saruman's plans until after Isengard's destruction and Pippin looks into the palantir. Sauron assumes the palantir was still in Isengard and sent a Nazgul out at once to collect the "treat," but as Gandalf smartly observes Saruman has finally caught himself in the vice:

"So Saruman will come to the last pinch of the vice vice that he has put his hand in. He has no captive to send. He has no Stone to see with, and cannot answer the summons. Sauron will only believe that he is withholding the captive and refusing to use the Stone."~The Palantir

Saruman was dominated through the Palantir, because he lost his "integrity", so to say, he abandoned the very purpose that the Istari were sent to Middle-earth. However, he was not dominated in the sense that he was Sauron's puppet, under his control. Saruman was able to not only deceive his former friends, but keep his true mind hidden from Sauron. (He sure let the Wiki know who was boss! ) and Sauron only found out of Saruman's betrayal until Pippin looks into the Palantir, which was even after Gandalf and all found out!

Of Saruman and Denethor, it was apparently Denethor who actually made first use of the Stone available to him.

It was believed after his wife died (2988) he began viewing the stone:

After her death Denethor became more grim and silent than before, and would sit long alone in his tower deep in thought, foreseeing that the assault of Mordor would come in his time. It was afterwards believed that needing knowledge, but being proud, and trusting in his own strength of will, he dared look into the palantir of the White Tower.~Appendix A: Gondor and the Heirs of Anarion; The Stewards

Saruman didn't begin to view the stone until the TA 3000 (according to Appendix B)