Confident enough to value a share at 10+ BTC.
10BTC per share sets the market cap of AM at 4 Million BTCs: With 21 Million BTCs ever in circulation, that's pushing it - The most likely option is we ride the bubble-train to those waters, then deflate at some bubble-defining-moment to more reasonable worth. That could be some big name getting in the game. Ask yourself how much you'd be willing to pay for an AM share if you just heard that Intel or AMD had started designing new chips specifically for mining. With the coming divs, remember to keep a cool head.
(Without anyone else in the game and those divs, I'd pay 6BTC. With AMD/Intel/competitor with strong product... 0.5 to 1BTC maybe?)
You have a point about the potential risks but the number of bitcoins in circulation shouldn't have any bearing on the potential valuation of a company. The total value of an economy isn't just the money in that economy but all of the goods you can buy or trade with it, including financial instruments such as debt. A higher valuation of ASICMINER adds to the total value of the bitcoin economy.