Why did he throw that?

[Nilpaurion Felagund]

The Palantir...

...themselves could be and usually were kept in the dark...

(UT, Part IV, The Palantir)

...were kept secret. During the time of Gondorian control over Isengard, access to them was limited...

Each Stone has its own warden...Other persons were appointed to visit the Stones, and Ministers of the Crown concerned with "intelligence" made regular and special inspections of them...

(UT, Part IV, The Palantir)

...which gives the impression they were locked in some secret chamber. Hence, Grima could not really find out that it existed, or at best, knows it but could not know what it was for. If he does, he knows it was very important.

Probably, at the coming of Gandalf and Co., he was surveying the Stone...no, wait...

[Sauron:] Why have you neglected to report for so long?

(LotR, Book III, Chapter 11)

...no, he could not be using the Stone. He was there for some other reason, probably a plan to escape or for a last strike...

Grima was probably in an anteroom, awaiting his master's orders. So it was he who frist heard Gandalf. Since Saruman turned his attention to his visitors, he may have left the door to the palantir room unlocked, thus allowing Grima access to the room.

Why Grima threw it? A few reasons spring to mind:
[LIST][*]Kill two birds with one stone, literally.

It glanced off the iron rail, even as Saruman left it, and passing close to Gandalf's head...

[*]Kill two birds with one stone, half literally.

Discussed already.
[*]Help the good guys.

No, don't laugh, don't kill me, just let me explain. See, Grima was a man of indecision. He switched from one side to the other with ease, depending on its appeal. Now Gandalf maybe an enemy, but a victorious one. He wants a part of that victory(maybe he regrets not fighting alongside Theoden...), however small, like freedom. He was angered by Saruman's stubborn refusal, and to "convince" him, threw the Stone to Gandalf. Since Saruman lost one of his "precious," a communication device at that, Orthanc was made less homely for him.

Later days! [img]smilies/cool.gif[/img]
->Elenrod

[ 1:20 AM November 27, 2003: Message edited by: Nilpaurion Felagund ]