If you have a serious amount of coins sitting in your wallets, and you declare these holdings when filling in your yearly tax reports, then they will definitely make you pay tax over what you hold in Bitcoin. As long as you don't declare any of your holdings, the government won't find out, and thus you end up escaping from paying tax. That's how it should be done.
Governments indeed won't find out what you are hiding, but the recent news of the IRS and Coinbase is just a first sign of what governments are going to do in the coming years. If we don't declare our coins, and I am sure the far majority won't be doing that, then the governments will come after us by forcing exchanges to hand over every bit of important user information. I just wonder what the next actions of the governments will be once they investigate user information. I think they will force people to pay tax plus a penalty for tax evasion. It's an extremely sad outcome if this turns out to be reality, but governments see tax evasion as a serious crime.
Yes, I think it's being done in my country already. We have a wallet/exchange here called coins.ph which I think is a subwallet of coinbase.
A user will have to go through verification processes - from ID, to address, phone, and business verifications - when signing up with them. Once you get verified on all these plus link your coinbase wallet, your cash in and withdrawal limits get upgraded to the highest possible amount of bitcoins and cash you can withdraw or deposit on a 24-hour basis.
They may also ask you to go through a skype interview if they aren't convinced by documents you've submitted. So anonymity of users don't actually exist here -- if you signed up with them.