Mikalhart is correct about generating random pin values to turn the LEDs on and off. That would be okay if you used six LEDS for each die to represent the pips on the die. Of course, then you would need only six pins for each die and you could generate a random number between 1 and 63 (or between 4 and 127 if you wanted to "save" the serial pins).
With one 7-segment LED, there would be a unique binary number conforming to each number 1 through 6. Depending on how you map your Arduino pins to the LED pins, the values for each number would differ. For example, if Arduino pin 3 is mapped to the top horizontal segment, pin 3 would be high on numbers 2, 3, 5 and 6.
At any rate, for my wiring, the LEDs for 1 would be set by PORTD=9, 2=94,, 3=31, etc.
So you could set up an array like:
byte pindata[7]={0.9,94,31,43,55,119};
Then get a random number between 1 and 6 in variable "value"
Then set PORTD=pindata[value];
I think in the end a combination of the array and the port control would be the most efficient.
Two problems, of course...we're messing with the serial pins by using the lower pins. Secondly, PORTD would not be portable to other microprocessors.