Did Gandalf make a mistake at Isengard ?

[The Might]

Boromir makes a good point of why he probably wouldn't have done in the first place, but as already stated even if he did inform him, there is no way a Nazgûl could do anything about it.
Proof?
The Nazgûl who flew over the camp on Dol Baran was not able do to anything. Now, indeed it did not directly respond to Pippin talking to Sauron through the palantir, the journey was too long and he had appeared immediately. He was actually sent to check on Saruman, but, even as its master detected Pippin and it was right above him it could not do anything about it.
So I doubt it could had done anything else sometime earlier.

[Ibrîniðilpathânezel]

Inziladun makes a good point, that Gandalf was not aware of Saruman's possession of a palantir until after it had been flung at him (contrary to what was shown in Jackson's version of the story). Moreover, when Pippin looked into the palantir, Sauron's first response was to think that it was Saruman reporting, which, for some reason, he had not done in some time. "So you have come back. Why have you neglected to report for so long?" From that reaction, I would hazard to guess that Saruman was no longer inclined to tell Sauron anything, unless it could save his skin, and especially not if he felt he might be able to use the information first, to his own advantage.

[Formendacil]

Originally Posted by Ibrîniðilpathânezel

Inziladun makes a good point, that Gandalf was not aware of Saruman's possession of a palantir until after it had been flung at him (contrary to what was shown in Jackson's version of the story).

Although the basic point here that Inziladun and Ibrin are making is to the effect that the palantír did not play a deliberative role in Gandalf's actions, I think it's incorrect to say that he did not know a palantír was in Saruman's possession.

The Orthanc-stone, after all, had always been at Isengard. Unlike the stones of Arnor, it was never moved due to enemy advances, and unlike Osgiliath and Minas Ithil, Orthanc was never taken by an enemy army--though, if my memory serves (and it IS hazy), the garrison did become strongly Dunlendish at some point, and resist Rohirric influence.

However, assuming that my memory IS right, then it also says that this was a huge part of the deliberative reasoning in Saruman receiving guardianship of Orthanc from the Stewards--Gondor wanted to know the palantír was safe in the hands of a wizard.

Granted, I doubt that the palantír was present much in Gandalf's thoughts--insofar as he knew, they weren't being used. It is hugely clear that the revelation of Gríma chucking the stone at him was of something he'd not known before... BUT... in the interests of pedantry, it seems fair to suggest Gandalf should have known it was there, even if he hadn't made the connection.

[Inziladun]

Originally Posted by Formendacil

Although the basic point here that Inziladun and {b]Ibrin[/b] are making is to the effect that the palantír did not play a deliberative role in Gandalf's actions, I think it's incorrect to say that he did not know a palantír was in Saruman's possession.

The Orthanc-stone, after all, had always been at Isengard. Unlike the stones of Arnor, it was never moved due to enemy advances, and unlike Osgiliath and Minas Ithil, Orthanc was never taken by an enemy army--though, if my memory serves (and it IS hazy), the garrison did become strongly Dunlendish at some point, and resist Rohirric influence.

If Gandalf was so certain of Saruman possessing the Stone, why would that not have been his first assumption of the manner in which Saruman and Sauron communicated?

There was some link between Isengard and Mordor, which I have not yet fathomed. How they exchanged news I am not sure; but they did so.

TT The Palantír

Surely, if Gandalf had been fully aware of the Orthanc-stone, it would not have taken Pippin's experience on Dol Baren for him to recognize what the Stone was.
Unlike Saruman, Gandalf's mind did not have a special affinity for artifacts of power, and I can see how the palantíri would have eluded his attention. It certainly appears to me that the event on Dol Baren took him completely by surprise.

[William Cloud Hicklin]

And how would a Nazgul have managed to seize a hobbit guarded by Ents, anyway?

[The Mouth of Sauron]

Originally Posted by William Cloud Hicklin

And how would a Nazgul have managed to seize a hobbit guarded by Ents, anyway?

A Nazgul could have snatched them easily enough, as they sat alone by the gate, while the Ents were working at the north side of Isengard.

[Hakon]

Originally Posted by The Mouth of Sauron

A Nazgul could have snatched them easily enough, as they sat alone by the gate, while the Ents were working at the north side of Isengard.

I think even the Nazgul fear the Ents, they would not venture towards Isengard if they were aware of the Ents there.

[PrinceOfTheHalflings]

Originally Posted by Formendacil

Although the basic point here that Inziladun and {b]Ibrin[/b] are making is to the effect that the palantír did not play a deliberative role in Gandalf's actions, I think it's incorrect to say that he did not know a palantír was in Saruman's possession.

The Orthanc-stone, after all, had always been at Isengard.

Gandalf probably did not know of the Orthanc-stone. Even Saruman did not know for certain that the Orthanc-stone was there until he moved into Isengard:

"Saruman had no doubt from his investigations gained a special knowledge of the Stones, things that would attract his attention, and had become convinced that the Orthanc-stone was still intact in its tower. He acquired the keys of Orthanc, nominally as warden of the tower and lieutenant of the Stewards of Gondor. At that time the matter of the Orthanc-stone would hardly concern the White Council. Only Saruman, having gained the favour of the Stewards, had yet made sufficient study of the records of Gondor to perceive the interest of the palantíri and the possible uses of those that survived; but of this he said nothing to his colleagues." (from Unfinished Tales)

So, in Tolkien's own words, only Saruman had much interest in, or knowledge of, the Stones and he never mentioned them to other members of the White Council. In other words: Gandalf probably did not know about the Orthanc-stone.