In major markets I think a low risk growth stock with a P/E of 10 is what most would consider a 'bargain.'
A low risk growth stock with a P/E of 10 would indeed be a bargain. But the catch here is that SatoshiDice is so easy to copy that it may not be a growth stock.
An astute observer may realize that by offering shares, it becomes an interesting bet to simply buy shares of SatoshiDICE instead of build it all over again and hope to outdo its marketing and brand exposure and business relationships. Not many understand that SatoshiDICE has achieved certain things which are very hard for new entrants to overcome, I won't go into all the details but examples like the 6 of the top 7 Bitcoin addresses, the massive word of mouth advertising, and integration with Blockchain.info's wallet (and soon, mobile app) are relevant.
You can spend a few thousand BTC to build a competitor and try to take a few percent market share, or you can spend a few thousand BTC and own a few percent of SatoshiDICE itself. If you throw in costs of time, dev resources/opportunity cost, statistical modelling and security risks which SatoshiDICE has already accomplished, and the decision shouldn't be too difficult.
Edit: Also, an astute observer will realize that after the IPO, SatoshiDICE will have a war chest with which to battle any competitors into submission.
If that was how the world worked people would cease to innovate and just 'invest' in what already exists......
That is not how the world turns.