I don't think you read the offer very carefully. 0.0032 BTC (lowest tier) buys someone 1/100M of the revenue of SatoshiDICE! The dividend yield works out (if worked out by people who actually can do division and multiplication) at an estimated 0.00036 BTC a year per share, or a little over 10%.
And if you have to pay 20btc just to register on MPEX just to get a few shares you will NEVER make a profit buying S.DICE.