The Phrase Finder describes the meaning to be, “An irrevocable choice has been made.” The expression refers to the rolling a die as a singular of dice. “Julius Caesar is supposed to have spoken this phrase when crossing the Rubicon,” although the Phrase Finder is a bit skeptical.  The earliest citation found was a 1626 quote from Sir Thomas Hebert, “Aiijb, Is the die cast, must At this one throw all thou hast gaind be lost?”