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.
Theoretically, ASICMiner could start selling hot dogs if it made business sense. Thus, the profits from the dividends are not constrained in any way by the number of bitcoins remaining to be mined.