habitually untalkative; temperamentally disinclined to talk
The usage note in the Webster’s Revised Unabridged Dictionary is helpful. Silent has reference to the act; taciturn, to the habit. A man may be silent from circumstances; he is taciturn from disposition.
It reminds me of the distinction between reticent and reluctant. The usage note in the American Heritage Dictionary is even more revealing: Taciturn implies unsociableness and a tendency to speak only when it is absolutely necessary.