In reply to:
I'd always thought it refered to the gambling cube or some fascimile thereof for the reaons you and others have cited. Interesting allusion for me. I've always thought that the emphasis was part "fait accompli" the done deed. He's done his part, flung die by crossing the rubicon - but the die has not yet settled, that is, he didn't at the time not know the outcome of his roll, his gamble. (Never knew it came down to us from Plutarch.)
Plutarch says it was proverbial. From Dryden's 1683 (yes that Dryden) translation http://classics.mit.edu/Plutarch/caesar.html
When he came to the river Rubicon, which parts Gaul within the Alps from the rest of Italy, his thoughts began to work, now he was just entering upon the danger, and he wavered much in his mind when he considered the greatness of the enterprise into which he was throwing himself. He checked his course and ordered a halt, while he revolved with himself, and often changed his opinion one way and the other, without speaking a word. This was when his purposes fluctuated most; presently he also discussed the matter with his friends who were about him (of which number Asinius Pollio was one), computing how many calamities his passing that river would bring upon mankind, and what a relation of it would be transmitted to posterity. At last, in a sort of passion, casting aside calculation, and abandoning himself to what might come, and using the proverb frequently in their mouths who enter upon dangerous and bold attempts, "The die is cast," with these words he took the river.
Suetonius just says (Rolfe's 1920s translation at: http://www.fordham.edu/halsall/ancient/suetonius-julius.html):
XXXII. As he stood in doubt, this sign was given him. On a sudden there appeared hard by a being of wondrous stature and beauty, who sat and played upon a reed; and when not only the shepherds flocked to hear him, but many of the soldiers left their posts, and among them some of the trumpeters, the apparition snatched a trumpet from one of them, rushed to the river, and sounding the war-note with mighty blast, strode to the opposite bank. Then Caesar cried: " Take we the course which the signs of the gods and the false dealing of our foes point out. The die is cast [ Iacta alea est,' inquit'].
Bingley
