[alatar]

When Gandalf is taken prisoner in Orthanc, I think that more is going on than what we read. Saruman sets Gandalf away as something of unknown value. He does not destroy Gandalf (even if he could) as Gandalf may have some information, or some other resource, that will be of use to Saruman, who, as we know, is ducking not only the White Council but also the Eye. Saruman has other more immediate tasks at hand, and so having it out with Gandalf may have to wait. This may be his rationale, as he truly knows that he cannot overcome Gandalf and remain unscathed (sans possessing the One).

Gandalf, having been in tight spots before, merely accepts his imprisonment. He is not sure if he can escape Orthanc, at least without risking killing Saruman, and although uncomfortable, he still is alive to fight another day. His fate may be to wait out the war, until either Sauron conquers or is vanquished. While he waits, surely he thinks of what he can do to aid the war. Or maybe he knows that his fate is not to rot atop Orthanc, and that this waiting on the sidelines is actually part of the bigger plan.