Origin and meaning of " The Die Has Been Cast"

“A decision has been made and the outcome cannot be changed” or "a point of no return"

By 49 BC Julius Caesar had significantly extended he Roman Empire. He had conquered Gaul and subdued the Celtic and Germanic tribes. During this period he had become very popular with the Roman people, his soldiers had great respect and loyalty for him, and he had amassed a large personal fortune.
This scared the Roman Senate who saw him as a threat to their power. So what they did is they ordered him to resign his command and disband his army. To ignore this command would make him an “Enemy of State”, which had dire consequences. Pompey, in Rome was given the task to ensure that this edict was enforced.
To make things worse there was a Roman law that forbade any general from crossing the Rubicon River and entering Italy proper, with an army. This would be interpreted as an act of treason. And it just so happened that Caesar was at Ravenna, just on the other side of the Rubicon when he was ordered to stand down.
Should he cross this river with his army his intentions would be known, and mark the point of no return, civil war would ensue.
Caesar took the decision to cross the Rubicon with his army, and face the consequences.
As they crossed the river Caesar cried out “ Let us go where the omens of the Gods and the crimes of our enemies summon us! THE DIE IS NOW CAST!”
"Alea iacta est"
(Source: Suetonius, Roman historian, 50-130 AD, based on documentation and interviews).
It would appear as though the Die referred to, was the singular form of a pair of Dice. In other words one the die had been thrown (cast), the result could not be changed, as in a gamble.
Crossing the Rubicon has the same meaning.
Later references refer to once molten metal is poured (cast) into a mould (die) the form cannot be changed, which is a practical explanation.
As with most of these old sayings there are different opinions as to which one was the correct one. I like the Caesar version

Edit : 7 November 2008
Verbatim from Suetonius
"As he stood in doubt (i.e. Caesar), 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 shepards 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 a mighty blast, strode to the opposite bank. Then Caesar cried; “Take we the course which signs of the gods and the false dealing of our foes point out. The die is cast (Iacta alea est)"
Suetonius, Da Vita Caesarum, Divus Iulius , paragraph XXXII