If it's $20,000 cash in the mail, it weighs about 200g (100 $100's) and to mail it about an hour from here (1 hour 30 minutes by car) Rochester to Buffalo, registered mail with $25,000 insurance, flat rate priority mail envelope is $53. I guess that price is the same across country.
OK, so it's actually cheaper to mail cash than to trade bank dollars for bitcoins. I'll get off my pulpit now.
(PS: I'm pretty sure you actually have to pay taxes on that cash you received by mail, especially if it's registered and insured, now that it's yours, it's income. Bitcoins on the other hand are not cash. I am a newbie to taxes, but I don't think you would have to pay tax on bitcoin income unless you were actually cashing them out. Maybe capital gains. Any experts?)
And then, only if you actually cashed out, right? If you leave them as bitcoins and the value drops below what you paid, now you've got a loss and maybe deduction instead...
It would make sense that you should pay taxes on mining income (if they can find you).
Most people earning income in bitcoin probably aren't doing it by mining, especially after ASIC arrives. Just to point out that mining is only one of many ways to earn bitcoin income.
I am not so sure that ASIC will kill the small-time mining scene, the low-end ASIC miners are cheaper and less power hungry than video cards (though you can't play games on them), it will just mean that if you want to mine bitcoins, you'll have to get one (or more), and you can't rely on your video card anymore. So a lot of people will be pushed out.