I will agree that every change to the hardware does open the door for the potential of a brute force attack.
However I think the "Could" has to be tempered by the "Would". Simply, ASIC's are shipping right now to legitimate miners and the hash rates are reflecting that. If you have followed Butterfly labs, I can tell you that the chance of ordering and receiving 300 units of that magnitude would require far more resources than just the 7.5M of cash (physically the ASIC's just are not available), so by the time someone could get that much hash power, they would need twice as much again.
But for the sake of argument, someone COULD get their hands on enough equipment and COULD overpower the network. Then they would not only have to outpace the blockchain building, but they would have to introduce something malicious to double spend or otherwise compromise the network. OK, lets say a government has all of this hardware, software and development power and has the will do to it, the end user community would only be "fooled" for a VERY short period of time (as soon as someone noticed a doublespend or other malice). At that point bitcoins would be worth nothing because everyone would know that they were hacked, so the entire effort probably could not have even repaid the cost of the hardware alone. To rectify the whole situation (and this has been done already), the blockchain could be forked by the "legitimate" development team, back to the point in time before the corruption began and let legitimate nodes and pools begin again with patched software and all of the previous bitcoins would be in tact.
It is a very real thing to consider, but I would be more afraid of Google Play being hacked and pushing out a corrupt wallet or other software that could easily constitute an economic majority in a matter of hours, and it would cost zero.
I dont think that the cost/risk/reward is there, since there are many other digital currencies and users could potentially just migrate (Mt Gox would be a perfect example)
Always good to keep doing the math though.
People do not tl;dr this. THIS right here is worth reading.
You bring up an excellent point. The one advantage Bitcoin has is that its history CAN be rewritten when 51% of the network agrees with you. Go back to before the attack happened and start from there again. If the attacker fixed his software as well, we could attempt to literally block his IP(s) but I don't know if that is possible from my understanding of p2p networking.