I use an iPod/iPhone Settlers Dice App. The benefits I see are:
1.) They're quiet,
2.) They're fast,
3.) They don't make a mess and get hung up on piles of cards or the edge of the board, and
4.) The probabilities are assured by the programming.
The probabilities exactly match the theoretical probability of each dice roll, which is not the case with physical dice.
I have used two iPod Touch/iPhone apps: "Settlers' Stats" and "Dice of Catan". "Stats" has a so called feature that allows you to track point changes of the various players. This turns out to be more trouble than counting again when you wonder. This app stinks. "Dice of Catan" is a different story.
"Dice of Catan" (On the App Store) costs $0.99. It is a very pleasant and simple app to use, and it takes the physical dice out of the game, which I think is a big plus. You tap the dice, and they display a pseudo-random (probability driven) Settlers of Catan roll.
Cities and Knights (C&K) support (has a die for barbarian movement/progress card.)
Safe mode for early rounds, which allows no sevens (and no barbarian movement in C&K).
Multicolored dice (one red, one green, see below.)
Tracks barbarian movement and has a pop-up reminder for the Robber/Bandit and when the Barbarian strikes.