Doing a home made ASIC is not a trivial.
Not just not trivial, totally impossible. You can
design an ASIC at home, but to produce it you need a multi billion dollar factory. If you bring enough dollars, and buy enough units, the companies owning those factories will happily produce them for you. That's what's happening with Avalon and BFL.
It's cool how most of the users have a trillion posts, but so few actually know and have been in a semiconductor manufacturing facility.
To design an ASIC you need a software which is expensive and you have to be really good specialist. It might not work otherwise.
And you need MILLIONS of dollars worth of equipment, not billions.
Companies, would not produce these miners because they could be adjusted for cellphone interception. The cellphone code is just 56bit. If you have a rainbow table it goes down to 36. You can imagine how easy would it be to get 10 of these devices and then decrypt conversations in real time.
And there is another issue - banks are not going to be happy that big companies are promoting a decentralized currency. It's just the way they work.