I think it's undeniably proven the amazing power of Bitcoin when applied to just one industry - online gambling.
It isn't just the power of bitcoin, it is the power of cryptography. The problem that SatoshiDICE solves is there was no previous easy to use online wagering service that is not only provably fair but gives an especially high payout compared to other online wagering services.
SatoshiDICE isn't the first to be provably fair though.
BitLotto was among the first to use the transaction hash in the blockchain as the value that determines whether the bet wins or not.
What SatoshiDICE did different though was it figured out how to do instant payouts. Because the payout uses for the transaction's input the wager transaction itself, there is no risk of paying out using funds that won't confirm. That was quite an innovation and now one million wagers later, it has proven its value.
There are a couple other "provably fair" wagering systems (including bitZino), but they don't all use a blockchain transaction hash so it is harder to verify. With SatoshiDICE, I can still go in a year from now for instance and use the blockchain plus the hash keys file to verify that every single wager ever made to SatoshiDICE was indeed processed properly as far as win/loss status and verify that the payouts were successfully sent.
Disruptive innovations are defined as being at least ten times better than existing solutions. With a 1.5% house advantage versus 4% to 8% or more for Vegas casinos, SatoshiDICE is pretty close to qualifying as one.