Basic math shows how overpriced the shares are: there are ~10million more bitcoins left to be mined over the next century or so. Even if ASICMiner were able to maintain 20% of the total network hashrate throughout that period (breathtakingly unlikely), they'd only mine 2million BTC. Split between the 400,000 shares that creates an upper limit of 5BTC/share.
You did forget the hardware sales
You forgot 2 things:
1) If a company is selling shovels to those looking for gold. Can the company make more than all the gold that exists in the world? Obviously YES. AsicMiner is selling shovels. I saw someone try and estimate the amount of hashing by asicminer hardware (sales + it's own) - i think it was more like 30% of total network speed.
2) Transaction fees. When bitcoin takes off we should see transaction fees drastically increase, thus the about 'mined' is infinite.