[Boromir88]

I can see Mr. Underhill's side of the argument. If we look at the quote in full:

'Then suddenly he seemed to see me, and he laughed at me. It was cruel. It was like being stabbed with knives. I struggled. But he said: "Wait a moment! We shall meet again soon. Tell Saruman that this dainty is not for him. I will send for it at once. Do you understand? Say just that!"~The Palantir

Before this moment Pippin tells Sauron that he's a Hobbit. What's interesting is that I don't think Sauron ever seen a Hobbit before this...he hadn't even heard of them until Gollum mentioned them. So, when Sauron comes to see Pippin, he's taken back and laughs. Then after he has his laughs for a while...Wait a moment! (something just came to him that is important) We shall meet again soon.. Sauron sees there is some importance that he might beable to get from this 'hobbit.'

I can further understand Mr. Underhill's argument as Sauron referring to Pippin as 'dainty' (something delectable, delicate...etc) and further dehumanizing him referring to Pippin as 'it,' because when Sauron first sees a hobbit he is filled with laughter...as in 'How is this pathetic creature going to harm me.'

Also, when Pippin is first recovering from the encounter with Sauron he mistakes Gandalf for Saruman:

'It is not for you, Saruman!' he cried in a shrill and toneless voice,' shrinking away from Gandalf. 'I will send for it at once. Do you understand? Say just that!'~Voice of Saruman

Pippin mistakes Gandalf for Saruman and shrinks away from him. Now Pippin is in a state of shock at this time, but either:

1) Sauron convinced Pippin that he had something valuable Sauron wanted (and hence Saruman had).

2) Pippin was reacting to what Sauron told him, and shrinks away from Saruman (well whom he believes to be Saruman), and reiterates Sauron's words 'It (or Pippin/he) is not for you, Saruman!'
Mithadan, if we take into account that he only wanted to send one Nazgul to go to Isengard, than I don't think Sauron thought he was going to get the Ring. You may be right, maybe Sauron did not know at this time that Saruman had planned to betray him (eventhough if Grishnakh did say 'Saruman is a fool: a dirty, treacerous fool, but the Eye is on him.'...there still could be some question as to whether Sauron knew what Saruman's plans were at this time. Though Sauron was definitely distrustful of him by this time, when he encounters Pippin in the Palantir). I would think though that if Sauron believed Pippin had the Ring...therefor Saruman had it, wouldn't it be more important to Sauron than just sending one Nazgul? I mean all 9 of them together failed to get the Ring, so I don't know how much faith Sauron would have in sending just one Nazgul into Isengard to get the Ring.

I guess another possibility could be that Sauron believed Saruman had the Ring, and since he had the Hobbit with him, it would prove useful to get both of them (maybe for information as far as what the 'West' was planning?) But, still I think if Sauron believed Saruman had a ring the situation would be more urgent than merely sending 1 Nazgul to go get it...even if Sauron believed Saruman was still his 'servant.'