It does not really make sense to put "Alea iacta est" in the plural form. The reason for this is that alea does not exactly mean "one dice" or "one die" (as some resources claim), but it in original Latin, alea is the name for the "game of dice" or "set of dice". As a consequence, using the plural would mean something like "The sets of dice were thrown" which is most probably not the intended meaning.
I found this explanation on the German wikipedia. It says:
Common incorrect translations are:
- „Aleum iactum est“ (assuming there is a singular aleum)
- „Alea iacta sunt“ (as above, but plural)
The word alea in the singular form means the "game of dice" as a whole. Consequently, "Alea iacta sunt" would be a correct plural, but would refer to multiple games (or sets) of dices. In fact, alea can be translated in the singular as wells as the plural form. However, in Latin the verb is always in the singular form.
The English wikipedia does not discuss the grammar of the English translation.
Also interesting (quoted from Wiktionary):
The form “the die is cast” is from the Latin iacta ālea est, a grammatically incorrect translation by Suetonius, 121 CE, of the Ancient Greek phrase of Menander ἀνερρίφθω κύβος (anerrhíphthō kúbos), which Caesar quoted in Greek (not Latin). The Greek translates rather as “let the die be cast!”, or “let the game be ventured!”.