So confused. Are you assuming everyone is evading tax with BTC? Because I'm not - I'm paying it. It takes some ninja accounting to do so correctly, but unless they drastically change the tax law retroactively, your scenario makes no sense.
I'm not saying anything about evading taxes.. I'm saying if the gov't wants to keep tract of its people's bitcoins.. and they happen to get in a Cyprus situation..Then they go after your bitcoins. I'd hate to have that situation.
As for not knowing who owns and who doesn't.. You probably did use a bank account to Gox or some other exchange for buying BTC. That pretty much links your identity with bitcoins and they can follow the trail via block chain, no?